There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it. If you are not allowed to move in any way (other than your eyes).
Where can you place an apple so that all but one person can see it?
Place the apple on one person's head.
Keep the apple on a persons' head. So all others can see it but only he himself can't see it.
On one person's head
llama, phoenix, hyena, alligator, beaver, elephant, tortoise, antelope