Reminds me of a joke that used to make me laugh out loud. (hmm easily amused, you may say...). Next time you're with someone, extend your hand, palm side up, and curl your fingers up slightly. Gesture to your hand and ask your friend "What's this" They won't know, and you answer (at the same time reversing your hand - ie palm side down - fingers still slightly curled) "a dead one of these". Keep your hand in that position, and ask them - "what's this" They won't know, or will answer incorrectly. The correct answer is "a live one of these". There's endless variation. One can be a live one or a dead one or a live one of the other or a dead one of the other and ther's no way for your friend to get the right answer. You and your friend can take it in turns to ask the questions. Believe me, there's great potential for hysterics. I know it sounds mad, but it was a slow news day at the time.
Along the lines of Rowan's joke -- You put your hand on your friend's head and flex your hand so the fingers slide back and forth and ask "What's this?" -- ? -- "A brain slug. What's it doing?" -- ? -- "Starving."