Post the hardest riddles you know. or can devise.

The more obscure, the better. It’s up to each poster whether to spoiler-box The solutions, but please provide them.

Two, courtesy of Richard Feynman:

Why does a mirror reverse right and left, but not top and bottom?

It does neither, but actually reverses front and back. That’s why words look reversed.

If you have an S-shaped sprinkler with holes on the inside curves, and connect it to a hose supplying pressurized water, the sprinkler turns away from the inside curves. Spinning counterclockwise, for example, if the sprinkler is oriented so that it looks like an “S” from above.

Suppose you took that whole hose/S-sprinkler system and placed it underwater. And instead of supplying pressurized water, you suck water through the hose like a straw instead. Which way does the sprinkler spin? Away from the holes? Or towards them?

Again, the answer is neither. The “suction sprinkler” doesn’t rotate. I’ll let someone else explain the physics behind it, though. Even Wikipedia isn’t helpful.

14 k of g in a f p d

Could you (or someone) explain this further?

(what does “front and back” mean vs. left/right?)

I would expect words looking reversed to be an example of right and left being reversed.


It means that, when you look into a mirror, objects on your left (right) are still on the left (right) in the mirror image, but objects closer to your eyes will be farther away in the mirror.

However, I would expect writing to look reversed no matter how you hold the mirror…

[spoiler]During normal operation, the momentum of the water is transferred from the sprinkler to the world. The sprinkler spins quickly one way, and the rest of the world spins slightly the other way. The two exactly cancel and angular momentum is preserved.

A reverse sprinkler doesn’t transfer momentum to the world, though. The water enters the sprinkler holes, but then impacts the rear part of the curved tube or is otherwise redirected to the center (where the fluid is removed). The momentum of the water is thus transferred only from one part of the sprinkler to another for a net of zero. You might have a small startup transient, but in the steady-state it doesn’t spin.[/spoiler]

Nice, but don’t forget our lovely Rebus.

What have I got in my pocket?


What is the smallest odd perfect number?

After millennia of trying, no one has ever found an odd perfect number, nor proven that one does not exist.

I don’t get that at all. Here are some explanations that make more sense to me.


(What’s ate, in colons?)

My contribution, from a book I read as a child. I can’t find a link to it, but it was in a story in which someone was captured by a riddle-crazed king who would let you go if you could create a riddle that would stump him. For some reason I associate it with Samson, although in Googling I find a different Samson riddle (in the same vein).

The riddle has stuck with me all these years later:

The answer:

The riddler was jogging–that is, walking and running. He saw a bird fly from the skull of an ox, in which it had built a nest. There were four chicks in the nest, and one egg unhatched.

Get two bottles of identical shampoo. Put them on a table so that their labels are facing eachother. Put a pane of glass between them. That is what’s happening with a mirror.

This isn’t difficult but I’m still surprised how some people get stumped. When they pull out pen and paper is when I start laughing:
A 30 year old man has a new born daughter. He is more than 30 times her age.

10 years later he is 40 and she is 10. Now he is but 4 times her age.

Five years after that he is 45 and his daughter 15. Now he is just 3 times her age.

Fifteen years later he is 60 and she is 30. Now he is only twice her age.

How many years would have to pass before they are equal age?

With apologies to A & C.

Does the Father’s age stop going up when he dies?

No, this isn’t it. The labels facing each other will not be mirrors of each other. I understand what is actually happening but I can’t put it in to words.

Try this. Imagine taking a photo of yourself and projecting it on to a piece of glass, now look at the image from behind. This is what is meant by reversing front and back. Or just imagine looking at an old negative film of something but from the back of the image, not the front.

That’s the kind of ballsed up logic I use to annoy my mother. When I was 10 you were 40, I was 1/4 your age. When I was 30 you were 60, I was 1/2 your age. I’m ageing twice as fast you are and will be dead before you even see a nursing home.

Hmm, let me do some reading…


  • i
  • flies

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]

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.