# Post the hardest riddles you know. or can devise. **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 9, 2018, 5:53pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593 "2018-07-09T17:53:01Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Mean\_Mr.Mustard](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mean_mr.mustard/32/3865_2.png) [@Mean\_Mr.Mustard](https://boards.straightdope.com/u/Mean_Mr.Mustard)\ **Post date:** [July 10, 2018, 9:53am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/21 "2018-07-10T09:53:06Z")
> [@Isamu](#): > > Hmm, let me do some reading… This passage, from Left **Hand of Dorkness** ’ link above, did the trick for me: > [@](#): > > The mirror does not reverse images from left to right, it reverses them from front to back relative to the front of the mirror. Stand facing a mirror. Point to one side. You and your mirror image are pointing in the same direction. Point to the front. Your mirror image is pointing in the opposite direction to you. Point upwards. You both point in the same direction. Now stand sideways on to the mirror and repeat. You are now pointing in opposite directions when you point sideways. Place the mirror on the floor and stand on it. This time you point in opposite directions when you point upwards and your upside down image points downwards. In all cases the direction reverses only when you point towards or away from the mirror. The answer stems from the fact that a reflection is not the same as a rotation. mmm ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [July 10, 2018, 9:53am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/22 "2018-07-10T09:53:42Z")
20 1 18 4 13 6 10 15 …? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [July 10, 2018, 11:45am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/23 "2018-07-10T11:45:09Z")
There is a compilation of Medieval riddles called the _Exeter Book of Riddles_\*, filled with the kind of things that show up in the “Riddle Game” between Gollum and Bilbo in _The Hobbit_. Tolkien undoubtedly had this book in mind when he wrote that chapter. There are three similar riddles in it that Tolkien didn’t dare use, because the answer is too obscure. Here’s one of them: > [@](#): > > I am greater than this middle-earth, > less than a hand-worm, > lighter than the moon, > swifter than the sun. > All the seas’ tides are in my embraces and the earthen breast, > the green fields. > I touch the foundations, > I sink under hell, > I soar over the heavens, > the glorious realm; > I reach wide over the homeland of angels; > I fill the earth abundantly, > the entire world and the streams of the oceans with myself. > Say what I am called.[ Give up? The answer is: [spoiler] Creation. That is, everything that God made. If you think that this kind of answer is “cheating”, well, so do I. I imagine some jester or lore-master, after having stumped the Lord of the Castle with one of these and grinningly revealing the answer being hauled away to the Dungeon or the Block by the irate Lord’s guards , and desperately crying out “I have another one, Lord! It’s a fair one, Lord! I SWEAR the answer’s not “Creation”!!!” [https://en.wikipedia.org/wiki/De\_creatura[/spoiler]](https://en.wikipedia.org/wiki/De_creatura%5B/spoiler%5D) - dating from the tenth century. There’s a Penguin edition translation of the hole thing, with answers and commentary. Sometimes the answers are only tentative. [Exeter Book - Wikipedia](https://en.wikipedia.org/wiki/Exeter_Book) ---
**Author:** ![Skald\_the\_Rhymer](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Skald\_the\_Rhymer](https://boards.straightdope.com/u/Skald_the_Rhymer)\ **Post date:** [July 10, 2018, 1:48pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/24 "2018-07-10T13:48:06Z")
> [@Thudlow\_Boink](#): > > What have I got in my pocket? Don’t make me call Manwë on you. ---
**Author:** ![solost](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/solost/32/11776_2.png) [@solost](https://boards.straightdope.com/u/solost)\ **Post date:** [July 10, 2018, 3:50pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/25 "2018-07-10T15:50:38Z")
Walking down a path on a very important journey, you come to a Y-shaped fork in the path. One direction is where you need to go, the other leads to certain death. There are 2 sentries, one standing next to the start of each of the 2 new paths. To learn which is the right path, you can ask one of the sentries one question. You know one of the sentries always tells the truth and one of the sentries always lies, but you don’t know which is which. What question do you ask? Ask either sentry “which direction would the other sentry tell me to go?”. Whatever they say, take the other direction. ---
**Author:** ![Busy\_Scissors](https://avatars.discourse-cdn.com/v4/letter/b/d78d45/32.png) [@Busy\_Scissors](https://boards.straightdope.com/u/Busy_Scissors)\ **Post date:** [July 10, 2018, 4:12pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/26 "2018-07-10T16:12:39Z")
The Blue-eyed islander one has to be up there as a ball-breaker of a riddle - sure it’s been discussed here before and is pretty well known on the internet. [https://xkcd.com/blue\_eyes.html](https://xkcd.com/blue_eyes.html) If it was just completely impenetrable then it wouldn’t be very interesting, but you read the solution and it makes sense - still impressed if anyone gets / got it straight up though. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [July 10, 2018, 4:42pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/27 "2018-07-10T16:42:33Z")
[quote=“septimus, post:20, topic:817593”] Here’s a classic that stumps most. A prison warden is ready to retire and to spend his final years on Tahiti. There are only four inmates left anyway, so he decides to just shut the prison down. But he can’t decide whether to let all the prisoners go free, or send them all to the firing squad. Instead of flipping a coin he devises one final sadistic game. Addressing all the prisoners together he says, "Tomorrow I’ll put a blob of paint on each of your foreheads, randomly choosing red or blue paint. Each of you will see the paint on the other three foreheads, but not your own. Feel free to discuss strategy now, but once the game starts tomorrow, if there’s any communication whatsoever among you, you all die. Tomorrow you will each have paper and pencil and will have a chance to guess your paint color. On the paper, secretly write ‘Red’, ‘Blue’, or ‘Pass.’ [ul][li] If all four of you Pass, all four will die.[/li][li] If anyone with Blue paint writes ‘Red’, all four will die.[/li][li] If anyone with Red paint writes ‘Blue’, all four will die.[/li][li] If at least one of you guesses correctly, and none of you guesses wrong, then all four of you will live.[/ul]The prisoners appoint you to devise their best strategy.[/li][/QUOTE] Are you sure you haven’t left a step out, like, knowing whether the person before you passed, or something like that? Because, as stated-- assuming the prisoners all write simultaneously-- i can’t see any way to communicate among the prisoners (even by something as simple as answering or not), which means I can’t see anything better than having prisoner #1 randomly write red or blue and the others passing. That gives a 50/50 shot at living or dying. Does the warden read and respond to the first prisoner before the second one has to write, maybe? ---
**Author:** ![BobLibDem](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/boblibdem/32/3149_2.png) [@BobLibDem](https://boards.straightdope.com/u/BobLibDem)\ **Post date:** [July 10, 2018, 4:50pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/28 "2018-07-10T16:50:46Z")
> [@septimus](#): > > Here’s a classic that stumps most. > > A prison warden is ready to retire and to spend his final years on Tahiti. There are only four inmates left anyway, so he decides to just shut the prison down. But he can’t decide whether to let all the prisoners go free, or send them all to the firing squad. Instead of flipping a coin he devises one final sadistic game. Addressing all the prisoners together he says, > "Tomorrow I’ll put a blob of paint on each of your foreheads, randomly choosing red or blue paint. Each of you will see the paint on the other three foreheads, but not your own. Feel free to discuss strategy now, but once the game starts tomorrow, if there’s any communication whatsoever among you, you all die. > Tomorrow you will each have paper and pencil and will have a chance to guess your paint color. On the paper, secretly write ‘Red’, ‘Blue’, or ‘Pass.’ > [ul][li] If all four of you Pass, all four will die.[/li][li] If anyone with Blue paint writes ‘Red’, all four will die.[/li][li] If anyone with Red paint writes ‘Blue’, all four will die.[/li][li] If at least one of you guesses correctly, and none of you guesses wrong, then all four of you will live.[/ul][/li]The prisoners appoint you to devise their best strategy. Seems to me that the best you can do is a 50/50 chance. Tell 3 to pass and the other takes a guess. ---
**Author:** ![Shoeless](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shoeless/32/7099_2.png) [@Shoeless](https://boards.straightdope.com/u/Shoeless)\ **Post date:** [July 10, 2018, 5:33pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/29 "2018-07-10T17:33:28Z")
> [@septimus](#): > > You enter an ancient tomb and find a red casket and a green casket. You manage to translate the four hieroglyphic inscriptions on display; they read: > (1) There is gold treasure in one of the caskets. > (2) There is gold treasure in the red casket. > (3) Exactly two of these four inscriptions are true. > (4) None of these inscriptions is true. > Where is the gold? > > [SPOILER]Did you guess that the gold was in the green casket? \*There was no physical force forcing the statements to form a coherent logic puzzle! \* The servants who placed the gold might have been illiterate or, knowing they were doomed, may have spitefully placed the treasure in the red casket. > > I think the servants swallowed the gold, hoping their families would be allowed to retrieve the bodies and thereby get rich![/SPOILER] My guess was that the gold treasure was in both caskets. But that answer depends on how you interpret the ambiguously worded (1). (1) There is gold treasure in one of the caskets. (FALSE, if you take it to mean _only_ one. But it could also mean _at least_ one, in which case it would be TRUE and blow up my solution.) (2) There is gold treasure in the red casket. (TRUE) (3) Exactly two of these four inscriptions are true. (TRUE) (4) None of these inscriptions is true. (FALSE) ---
**Author:** ![Dendarii\_Dame](https://avatars.discourse-cdn.com/v4/letter/d/bbce88/32.png) [@Dendarii\_Dame](https://boards.straightdope.com/u/Dendarii_Dame)\ **Post date:** [July 10, 2018, 7:07pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/30 "2018-07-10T19:07:28Z")
What builds up castles And tears down mountains Makes some people blind And helps others to see? Sand. As in sand castles, erosion, sandstorms, and eyeglasses, respectively. I remember this one from a Super Friends cartoon. ---
**Author:** ![Dendarii\_Dame](https://avatars.discourse-cdn.com/v4/letter/d/bbce88/32.png) [@Dendarii\_Dame](https://boards.straightdope.com/u/Dendarii_Dame)\ **Post date:** [July 10, 2018, 7:44pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/31 "2018-07-10T19:44:15Z")
And here’s one I read in a book, but I don’t remember the author. It’s at the beginning of eternity, The end of time and space, The beginning of every end, And the end of every race. The letter E ---
**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\ **Post date:** [July 10, 2018, 8:02pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/32 "2018-07-10T20:02:07Z")
> [@Thudlow\_Boink](#): > > What have I got in my pocket? Not again… ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [July 10, 2018, 8:28pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/33 "2018-07-10T20:28:54Z")
> [@septimus](#): > > Here’s a classic that stumps most. > > A prison warden is ready to retire and to spend his final years on Tahiti. There are only four inmates left anyway, so he decides to just shut the prison down. But he can’t decide whether to let all the prisoners go free, or send them all to the firing squad. Instead of flipping a coin he devises one final sadistic game. Addressing all the prisoners together he says, > "Tomorrow I’ll put a blob of paint on each of your foreheads, randomly choosing red or blue paint. Each of you will see the paint on the other three foreheads, but not your own. Feel free to discuss strategy now, but once the game starts tomorrow, if there’s any communication whatsoever among you, you all die. > Tomorrow you will each have paper and pencil and will have a chance to guess your paint color. On the paper, secretly write ‘Red’, ‘Blue’, or ‘Pass.’ > [ul][li] If all four of you Pass, all four will die.[/li][li] If anyone with Blue paint writes ‘Red’, all four will die.[/li][li] If anyone with Red paint writes ‘Blue’, all four will die.[/li][li] If at least one of you guesses correctly, and none of you guesses wrong, then all four of you will live.[/ul][/li]The prisoners appoint you to devise their best strategy. Do they all answer at the same time? If not, first two pass immediately. Third person passes immediately if the 4th has red and passes later if the 4th person has blue. 4th person answers corresponding to what the 3rd person did. ---
**Author:** ![kk\_fusion](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kk_fusion/32/15276_2.png) [@kk\_fusion](https://boards.straightdope.com/u/kk_fusion)\ **Post date:** [July 10, 2018, 8:57pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/34 "2018-07-10T20:57:25Z")
> [@septimus](#): > > Here’s a classic that stumps most. > > A prison warden is ready to retire and to spend his final years on Tahiti. There are only four inmates left anyway, so he decides to just shut the prison down. But he can’t decide whether to let all the prisoners go free, or send them all to the firing squad. Instead of flipping a coin he devises one final sadistic game. Addressing all the prisoners together he says, > "Tomorrow I’ll put a blob of paint on each of your foreheads, randomly choosing red or blue paint. Each of you will see the paint on the other three foreheads, but not your own. Feel free to discuss strategy now, but once the game starts tomorrow, if there’s any communication whatsoever among you, you all die. > Tomorrow you will each have paper and pencil and will have a chance to guess your paint color. On the paper, secretly write ‘Red’, ‘Blue’, or ‘Pass.’ > [ul][li] If all four of you Pass, all four will die.[/li][li] If anyone with Blue paint writes ‘Red’, all four will die.[/li][li] If anyone with Red paint writes ‘Blue’, all four will die.[/li][li] If at least one of you guesses correctly, and none of you guesses wrong, then all four of you will live.[/ul][/li]The prisoners appoint you to devise their best strategy. That was surprisingly difficult, but I finally got something. If you see three of the same color, say the other color. If you see two red and one blue, say blue. If you see two blue and one red, pass. You’ll win 10 out of 16. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 10, 2018, 9:35pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/35 "2018-07-10T21:35:00Z")
> [@septimus](#): > > The prisoners appoint you to devise their best strategy. [spoiler]First rule: if they see a pattern that could be RRRR or BRBB (using some prearranged order), then vote the _opposite_ of that. The prisoners will obviously die on a RRRR or BRBB, but live on a RRRB, RRBR, RBRR, BRRR, RRBB, BBBB, BRRB, and BRBR. Second rule, which _only_ applies to prisoners 3-4: if they see a pattern that could be RBBB or BBRR, vote the opposite. RBBB and BBRR combinations die, but RBRB, RBBR, BBRB, and BBBR live. That’s 12/16 chances to live.[/spoiler] ---
**Author:** ![Left\_Hand\_of\_Dorkness](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/left_hand_of_dorkness/32/7156_2.png) [@Left\_Hand\_of\_Dorkness](https://boards.straightdope.com/u/Left_Hand_of_Dorkness)\ **Post date:** [July 10, 2018, 10:05pm UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/36 "2018-07-10T22:05:56Z")
> [@solost](#): > > Walking down a path on a very important journey, you come to a Y-shaped fork in the path. One direction is where you need to go, the other leads to certain death. > > There are 2 sentries, one standing next to the start of each of the 2 new paths. To learn which is the right path, you can ask one of the sentries one question. You know one of the sentries always tells the truth and one of the sentries always lies, but you don’t know which is which. What question do you ask? > > Ask either sentry “which direction would the other sentry tell me to go?”. Whatever they say, take the other direction. If you assume that the sentries are rigorous rules-followers, there’s another question you can ask, and you can even ask it of a lone rigorously rule-following sentry who randomly will answer truthfully or falsely. I read this one as a kid and was fascinated by its devious answer. See if you can figure it out before clicking on the spoiler! [spoiler]If you were to answer the question “Is the left path the way to certain death” as truthfully as you’re answering this question, would your answer be “yes”? Imagine the left path leads to certain death. A truth-teller would answer, “Is the left path the way to certain death?” with “Yes.” A liar would answer, “No.” You’re asking if they’d answer “yes” to that question. A truth-teller would say “yes,” because they would. A liar would answer that question “no,” so when you ask if they’d say yes, they’ll lie and say “yes.” Similar logic applies if the left path doesn’t lead to certain death. This nested question allows you to cancel out lies with a lie-about-the-lie, getting a truthful answer every time.[/spoiler] ---
**Author:** ![UltraVires](https://avatars.discourse-cdn.com/v4/letter/u/ecccb3/32.png) [@UltraVires](https://boards.straightdope.com/u/UltraVires)\ **Post date:** [July 11, 2018, 12:06am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/37 "2018-07-11T00:06:53Z")
> [@kk\_fusion](#): > > That was surprisingly difficult, but I finally got something. > > If you see three of the same color, say the other color. > If you see two red and one blue, say blue. > If you see two blue and one red, pass. > > You’ll win 10 out of 16. > [@Dr.Strangelove](#): > > [spoiler]First rule: if they see a pattern that could be RRRR or BRBB (using some prearranged order), then vote the _opposite_ of that. The prisoners will obviously die on a RRRR or BRBB, but live on a RRRB, RRBR, RBRR, BRRR, RRBB, BBBB, BRRB, and BRBR. > > Second rule, which _only_ applies to prisoners 3-4: if they see a pattern that could be RBBB or BBRR, vote the opposite. RBBB and BBRR combinations die, but RBRB, RBBR, BBRB, and BBBR live. > > That’s 12/16 chances to live.[/spoiler] Isn’t this a gambler’s fallacy or reverse Monty Hall problem? IOW, it does matter if I see 3 red or 3 blue (or for that matter, 100 red or blue in a scenario with 100 prisoners). If it is a random process, the chances of me having either red or blue remain 50/50…right? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 11, 2018, 12:27am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/38 "2018-07-11T00:27:53Z")
> [@UltraVires](#): > > Isn’t this a gambler’s fallacy or reverse Monty Hall problem? [spoiler]I don’t think that’s the right way of looking at it. Well, at least the analogy doesn’t make sense to me. The first thing is that you want to concentrate badness. It’s true that for any individual’s guess they’ll be right only half the time. But what if you could make it so that all four prisoners were wrong at the same time, but when they were right it was at different times? And barring that, at least have more than one prisoner be bad simultaneously. When they guess the opposite of RRRR, if RRRR comes up, then they’ll all guess and all be wrong. But if BRRR comes up, then only one individual makes a guess and she’ll be right. If RBRR comes up, then a different prisoner will be right. Etc. We get a 4x improvement because the good guesses are spread out. Beyond that, there are similarities to error coding. Looking up Hamming distance would be a start. Basically, you want to find several “loci” and the set of combinations exactly one color-flip away. They can’t overlap or there will be ambiguity. The “clean” setup would have three 4-color combos and the 12 additional combos that are a step away. This still only gets you 12 wins but doesn’t require different rules for different prisoners. However, three combos like this don’t exist. What we can do instead is have two 4-color combos, with their 8 attendant color-flipped versions, and then two additional combos but only with two flipped versions for each. One possible set (in binary, since that’s easiest for me) is: 0000 -\> 0001, 0010, 0100, 1000 1011 -\> 1010, 1001, 1111, 0011 0111 -\> 0110, 0101 1100 -\> 1101, 1110 There are lots of possible combinations; that was just the first I found.[/spoiler] ---
**Author:** ![Mahaloth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mahaloth/32/1079_2.png) [@Mahaloth](https://boards.straightdope.com/u/Mahaloth)\ **Post date:** [July 11, 2018, 12:39am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/39 "2018-07-11T00:39:59Z")
> [@Isamu](#): > > flammeus > > - i > =fliammeus > > - flies > =ammu > +[pepper]? > =? No, it has no answer. It went on for years and I finally called the store and spoke with them. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 11, 2018, 12:47am UTC](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593/40 "2018-07-11T00:47:44Z")
BTW:[spoiler]If you have exactly 2[sup]n[/sup]-1 prisoners, you can always come up with an algorithm that saves you (1 - 1/2[sup]n[/sup]) of the time. So for 7 prisoners, you can live 87.5% of the time (and can’t do any better). I’m not sure of a general algorithm for other numbers but it will be at most floor(2[sup]N[/sup] / (N+1)) \* N / 2[sup]N[/sup].[/spoiler] [Previous page](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593.md?page=1) [Next page](https://boards.straightdope.com/t/post-the-hardest-riddles-you-know-or-can-devise/817593.md?page=3)