# Most difficult riddle you know(and the answer)? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [April 28, 2002, 11:10pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176 "2002-04-28T23:10:43Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![glee](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@glee](https://boards.straightdope.com/u/glee)\ **Post date:** [April 30, 2002, 10:41pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/41 "2002-04-30T22:41:57Z")
> [@](#): > > \*Originally posted by DMark \* > “Speak, friend, and the door will open.” > > What magical phrase will you say to get in? > > (Apologies to Tolkien fans, I sorta paraphrased it above. > It’s a riddle in the Hobbit. > Gotta read the book to get the answer or see Lord of the Rings first 10 minutes of the film.) The runes on the doors of Moria are in Elvish, so one says “Mellon” (Elvish for friend). This happens in the Fellowship of the Ring, not the Hobbit. It’s not the first 10 minutes of the film either. We may have to send the Watcher in the Water to haunt your dreams… ---
**Author:** ![Revtim](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/revtim/32/1042_2.png) [@Revtim](https://boards.straightdope.com/u/Revtim)\ **Post date:** [April 30, 2002, 11:18pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/42 "2002-04-30T23:18:13Z")
Even if you ask “How many of each animal did **Noah** take on the ark?” the answer isn’t so clear cut. One place in the bible says two of each, another says something like seven of each. ---
**Author:** ![Yo\_La\_Tengo](https://avatars.discourse-cdn.com/v4/letter/y/41988e/32.png) [@Yo\_La\_Tengo](https://boards.straightdope.com/u/Yo_La_Tengo)\ **Post date:** [May 1, 2002, 1:39am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/43 "2002-05-01T01:39:43Z")
Ok, nice job, **gatopescado**. Let’s see if anyone can get this one… _First think of the person who lives in disguise, Who deals in secrets and tells naught but lies. Next, tell me what’s always the last thing to mend, The middle of middle and end of the end? And finally give me the sound often heard During the search for a hard-to-find word. Now string them together, and answer me this, Which creature would you be unwilling to kiss?_ And if you read _Harry Potter_, you can’t answer this question. I’d like to see if anyone can actually answer this w/o ever hearing it before… ---
**Author:** ![Snooooopy](https://avatars.discourse-cdn.com/v4/letter/s/ee7513/32.png) [@Snooooopy](https://boards.straightdope.com/u/Snooooopy)\ **Post date:** [May 1, 2002, 2:48am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/44 "2002-05-01T02:48:10Z")
> [@](#): > > \*Originally posted by Yo La Tengo \* > \*\*Ok, nice job, **gatopescado**. Let’s see if anyone can get this one… > _First think of the person who lives in disguise, > Who deals in secrets and tells naught but lies. > Next, tell me what’s always the last thing to mend, > The middle of middle and end of the end? > And finally give me the sound often heard > During the search for a hard-to-find word. > Now string them together, and answer me this, > Which creature would you be unwilling to kiss?_ > \*\* Ass. I don’t want to kiss anyone’s ass. ---
**Author:** ![TheOtherOne](https://avatars.discourse-cdn.com/v4/letter/t/dbc845/32.png) [@TheOtherOne](https://boards.straightdope.com/u/TheOtherOne)\ **Post date:** [May 1, 2002, 2:56am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/45 "2002-05-01T02:56:08Z")
The man who builds it doesn’t want it The man who buys it doesn’t need it and the man who needs it doesn’t know it… So what is it? ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [May 1, 2002, 3:20am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/46 "2002-05-01T03:20:16Z")
> [@](#): > > \*Originally posted by Yo La Tengo \* > _First think of the person who lives in disguise, > Who deals in secrets and tells naught but lies. > Next, tell me what’s always the last thing to mend, > The middle of middle and end of the end? > And finally give me the sound often heard > During the search for a hard-to-find word. > Now string them together, and answer me this, > Which creature would you be unwilling to kiss?_ Most of the lines suggest the answer is “D”, but the “unwilling to kiss” is unclear. ---
**Author:** ![DoperChic](https://avatars.discourse-cdn.com/v4/letter/d/a9a28c/32.png) [@DoperChic](https://boards.straightdope.com/u/DoperChic)\ **Post date:** [May 1, 2002, 3:25am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/47 "2002-05-01T03:25:45Z")
> [@](#): > > \*Originally posted by TheOtherOne \* > \*\*The man who builds it doesn’t want it > The man who buys it doesn’t need it > and the man who needs it doesn’t know it… > > So what is it? \*\* A coffin. ---
**Author:** ![rjk](https://avatars.discourse-cdn.com/v4/letter/r/ed655f/32.png) [@rjk](https://boards.straightdope.com/u/rjk)\ **Post date:** [May 1, 2002, 3:43am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/48 "2002-05-01T03:43:56Z")
> [@](#): > > \*Originally posted by ultrafilter \* > \*\* > > Exactly what it says. You keep rolling the die until it comes up six. Maybe you roll it the first time and it comes up six; maybe you have to wait for a hundred times. Keep doing this as an experiment, and the average number of times you roll it will be six. > > \*\* How about: How many people do you need in a room before there’s a 50% chance that two of them have the same birthday? (The answer is somewhere in the 20’s, but I don’t recall it exactly.) \*\* [/QUOTE] 1. \*\* [/QUOTE] I haven’t done the arithmetic, but 23 is about right, IIRC. On the other hand, I did work out the die question, and six isn’t right. The problems have very similar solutions. The general idea is to take the probability of _failing_ on a single trial, and use that to get the odds for each successive try. For the whole series, we multiply the individual probabilities. So in the second problem we look for unique birthdays. The first person is ceretain to be unique, probability 1. The second is unique when he has a different b’day, 364/365 (ignoring leap years). The third is unique with probability 363/365, and so on. Multiplying as we go, the chance of all being unique drops below 50% between trial 23 and 24 (again, IIRC). Similarly, we look at the odds of _not_ rolling a six on a single trial, 5/6. Since the trials are independent, unlike the birthday problem, the odds don’t change for successive trials, and we keep multiplying by 5/6. Working it out, the chance of _not_ rolling a six drops below 50% on the fourth roll, so we expect a six by then. ---
**Author:** ![Papamurf](https://avatars.discourse-cdn.com/v4/letter/p/90db22/32.png) [@Papamurf](https://boards.straightdope.com/u/Papamurf)\ **Post date:** [May 1, 2002, 3:58am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/49 "2002-05-01T03:58:21Z")
> [@](#): > > **First think of the person who lives in disguise, > Who deals in secrets and tells naught but lies. > Next, tell me what’s always the last thing to mend, > The middle of middle and end of the end? > And finally give me the sound often heard > During the search for a hard-to-find word. > Now string them together, and answer me this, > Which creature would you be unwilling to kiss?** spider or perhaps spyder (I think) ---
**Author:** ![knock\_knock](https://avatars.discourse-cdn.com/v4/letter/k/22d042/32.png) [@knock\_knock](https://boards.straightdope.com/u/knock_knock)\ **Post date:** [May 1, 2002, 4:11am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/50 "2002-05-01T04:11:19Z")
> [@](#): > > _originall posted by rjk:_ > **Similarly, we look at the odds of not rolling a six on a single trial, 5/6. Since the trials are independent, unlike the birthday problem, the odds don’t change for successive trials, and we keep multiplying by 5/6. Working it out, the chance of not rolling a six drops below 50% on the fourth roll, so we expect a six by then.** **ultrafilter** is asking a different question - he wants to know the mean number of times you have to roll the die to get a six, not the median. The expected number of rolls can be expressed as 1_p(6 on first roll) + 2_p(6 on 2nd roll) + 3\*p(6 on 3rd roll) + … where p(6 on 2nd roll) represents the probability of getting a 6 on the 2nd roll and not before. You could sum the series, but it’s not as simple as it looks. Instead, think of it this way: if you roll the die many times, you expect to get a six one sixth of the time. So on average, every sixth roll will be a six. (btw, **rjk** , odds does not mean the same thing as probability. If the chance or the probability of something is 5/6, the odds of it happening are 1:5, and the odds against are 5:1) ---
**Author:** ![berdollos](https://avatars.discourse-cdn.com/v4/letter/b/ed8c4c/32.png) [@berdollos](https://boards.straightdope.com/u/berdollos)\ **Post date:** [May 1, 2002, 4:13am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/51 "2002-05-01T04:13:44Z")
and never leave and other leave who never entered? A hospital (the dead and the newborn) ---
**Author:** ![rjk](https://avatars.discourse-cdn.com/v4/letter/r/ed655f/32.png) [@rjk](https://boards.straightdope.com/u/rjk)\ **Post date:** [May 1, 2002, 6:59am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/52 "2002-05-01T06:59:55Z")
> [@](#): > > You have a standard six-sided die (i.e., equal probability of each number coming up). How many times do you expect to have to roll it before you get a six? Although it’s certainly a long time since I took stats (and I didn’t even come close to acing it), I still expect four rolls. Maybe I just don’t understand, or maybe I’m misreading the problem. I’ll go dig up a stats book someplace. (And I do know the difference between odds and probability; I just let myself get sloppy. And I should have previewed!) ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [May 1, 2002, 3:19pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/53 "2002-05-01T15:19:30Z")
The easiest way to solve the die problem is by conditional expectation. The trials are independent, so at each one, you can forget what happened before. Let p be the probability of rolling a six (p = 1/6, but it’s less typing if I use p), and let m be the mean number of times you have to roll the die to get a six. If you get it on the first try (with probability p), m = 1. Otherwise, you now expect to roll the die m + 1 times before you see a six. So by the formula for conditional expectation, m = p + (m + 1)(1 - p). Solve that, and you’ll get 1/p, which is equal to six. It’s possible to do this problem using the geometric distribution and some fancy manipulations of series, but that’s hard, and not worth the time. ---
**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:** [May 1, 2002, 4:35pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/54 "2002-05-01T16:35:15Z")
I remember when I was a kid reading this riddle somewhere. Is it attributed to Samson? I can’t remember: supposedly, some famous schmoe who was imprisoned asked this riddle and was freed based on the inability of his captors to answer it. Anyway: _As I walked and as I ran Out of the dead the living came. Four there were, and five to be; Answer this riddle or set me free._ ANSWER: As the narrator jogged along, he saw a cow’s skull lying by the road. A bird had nested in it and flew out, leaving behind a nest with four chicks and one egg. My favorite riddle as a kid: Q. How do you spell “hard butter” with four letters? A. G-O-A-T. My favorite riddles as an adult: Q. What’s so delicate that even saying its name can break it? A. Silence. Q. How many legs does a dog have, if you call its tail a leg? A. Four (calling a tail a leg doesn’t make it a leg) Daniel ---
**Author:** ![Yo\_La\_Tengo](https://avatars.discourse-cdn.com/v4/letter/y/41988e/32.png) [@Yo\_La\_Tengo](https://boards.straightdope.com/u/Yo_La_Tengo)\ **Post date:** [May 1, 2002, 6:59pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/55 "2002-05-01T18:59:07Z")
**Papamurf** got the answer to my riddle. Nice job. (Spy+d+er = Spider) ---
**Author:** ![askol](https://avatars.discourse-cdn.com/v4/letter/a/53a042/32.png) [@askol](https://boards.straightdope.com/u/askol)\ **Post date:** [May 1, 2002, 8:49pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/56 "2002-05-01T20:49:09Z")
Ah hell. I just wrote a post twice dealing with the expected value of dice question, using the expected value of a geometric random value with p=1/6, and it turned out to be six. I would show you the excruitiating deatail, but I’ve already written it twice. askol ---
**Author:** ![pravnik](https://avatars.discourse-cdn.com/v4/letter/p/8edcca/32.png) [@pravnik](https://boards.straightdope.com/u/pravnik)\ **Post date:** [May 2, 2002, 7:51am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/57 "2002-05-02T07:51:58Z")
> [@](#): > > _Originally posted by TN_hippie \* > \*\*You are in a room that is inescapable, except for two identical doors. One door leads to instant death, the other leads to freedom. You don’t know which is which. There are two other people in the room. They know all the answers. One of them always lies; the other always tells the truth. You don’t know which is which. You may ask **one** question. You may direct it to **one** person. > > What do you do? > > I’ll give you a few minutes. \*\* Ask either one “If I asked that other guy over there which door was the safe door, what would he say”? Then walk through the door other than the one he said. Another, from Lewis Carrol: on the island of knights and knaves, knights always tell the truth and knaves always lie. You walk up on Al, Bill and Charlie. Al says “All of us are knaves”. Bill says “Exactly one of us is a knight”. What are Al, Bill, and Charlie? ---
**Author:** ![Monocracy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monocracy/32/4493_2.png) [@Monocracy](https://boards.straightdope.com/u/Monocracy)\ **Post date:** [May 2, 2002, 9:31am UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/58 "2002-05-02T09:31:14Z")
> [@](#): > > posted by **ultrafilter** : > You have a standard six-sided die (i.e., equal probability of each number coming up). How many times do you expect to have to roll it before you get a six? > [@](#): > > posted by **knock knock** : > > > [@](#): > > > > originall posted by rjk: > > Similarly, we look at the odds of not rolling a six on a single trial, 5/6. Since the trials are independent, unlike the birthday problem, the odds don’t change for successive trials, and we keep multiplying by 5/6. Working it out, the chance of not rolling a six drops below 50% on the fourth roll, so we expect a six by then. > > ultrafilter is asking a different question - he wants to know the mean number of times you have to roll the die to get a six, not the median. Actually, to me, the question is pretty unclear. By it’s wording, it sounds like he’s asking for the **mode** , not the **mean** or **median**. And to that, the answer would be 1. ---
**Author:** ![glee](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@glee](https://boards.straightdope.com/u/glee)\ **Post date:** [May 2, 2002, 1:57pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/59 "2002-05-02T13:57:04Z")
> [@](#): > > \*Originally posted by pravnik \* > Ask either one “If I asked that other guy over there which door was the safe door, what would he say”? Then walk through the door other than the one he said. Coooee! I already answered that! > [@](#): > > \*Originally posted by pravnik \* > Another, from Lewis Carrol: on the island of knights and knaves, knights always tell the truth and knaves always lie. You walk up on Al, Bill and Charlie. Al says “All of us are knaves”. Bill says “Exactly one of us is a knight”. What are Al, Bill, and Charlie? Al must be a knave - because if he’s a knight, then his statement makes him a knave. Bill must be a knight - because if he’s a knave, then Charlie must be a knight (otherwise Al is telling the truth), but then Bill would be telling the truth. Since Bill is a knight, his statement means Charlie is a knave. ---
**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:** [May 2, 2002, 2:45pm UTC](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176/60 "2002-05-02T14:45:56Z")
Another answer to the two doors, one-person-tells-the-truth-and-one-person-lies question: **If you were to answer the question, “Does the left door lead to safety,” as honestly as you’re answering this question, would you answer affirmatively?** If the person is a truth-teller, then their answer to this question will be truthful, and you can trust it. If the person is a liar, then their answer to this question will contain a double negative (basically, they’ll have to lie about what the lying answer would be). Again, their answer will be accurate. This convoluted solution works for any logic puzzle in which you ask yes/no questions that are answered either truthfully or nontruthfully, as long as the answerers have a strong grasp of their logical duties. Daniel [Previous page](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176.md?page=2) [Next page](https://boards.straightdope.com/t/most-difficult-riddle-you-know-and-the-answer/106176.md?page=4)