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Re:Robert Krampf's response
This illusion is caused by persistence of vision.   As you move the top coin
to the end of its path in one direction, it pauses for a second as you reverse
direction.   Your eye continues to register the image for a fraction of a
second after the coin is miving in the other direction.   Before the image can
fade, the bottom coin has moved into the same position and the image continues.
 You see one coin against your finger tip, one coin against your thumb and a
third coin, whose image is made up of the combined afterimages of both top and
bottom coin.


When dealing with persistence of vision, you usually need to mention the phi effect as well, which is how the brain processes pictures shown quickly in succession.

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