The riddle
What Question Can You Never Answer Yes to?
Why yes cannot work
To answer "Are you asleep?" a person must speak, and a person who is speaking is awake. So "yes" is false the moment it is said. A person who really is asleep says nothing sensible, so the question can only be answered truthfully with "no" or with silence. The question is built so that the act of replying defeats the reply. It works on anyone, at any age, because it depends only on being able to answer at all.
What about sleep talking?
Some people do speak while asleep, a habit called somniloquy. It is common, especially in children, and usually harmless. What comes out tends to be mumbled or muddled, not a considered reply. The riddle is about answering on purpose, and a sleeper's murmur does not count as that.
Testing the guesses
A good way to run this riddle is to ask the group to try each guess by saying yes to it. "Do you like pizza?" passes, so do "Is it Tuesday?" and "Are you hungry?". Saying each answer out loud makes the clash easy to hear. Only one kind of question fails the test, and listeners often find it by asking what has to be true in order to reply at all.
The mirror image
There is a reverse version. "Are you speaking?" cannot be answered with an honest "no" out loud, since saying it is speaking. Together the two questions show how a few words can undo themselves.