Riddle: There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it without turning his head or body, or moving in any way (other than the eyes). Where can you place an apple so that all but one person can see it?
There are 20 people in the room and everyone can see others. And there are no items to hide the apple beneath or behind since the room is empty.
Where do you put the apple for 19 people to see and 1 person not to see?
Well… If we put it behind a person, we might block other people to see it. Because we don’t know the exact spots they are in.
However, what if we put the apple on top of one person’s head? In this case, we are making sure that everyone else except one person will see the apple!
Answer: Place the apple on one person’s head.