There are 20 people in an empty square room

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?

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Keep the apple on a persons' head.
So all others can see it but only he himself can't see it.