Hole in the Hand
Hold a cardboard tube (one from narrow kitchen towel rolls is ideal) against your right eye and hold the palm of your left hand up so that it faces you and touches the left hand end of the tube. Then, slowly move you left hand alongside the tube towards you face and notice what happens. Has a hole appeared in the middle of your left hand?
The inside story
Like all primates, our brains are programmed to cope with stereoscopic vision — complementary signals from two eyes. The brain merges the signals it receives from each eye, always assuming each signal will be of the same thing. The Hole in the Hand activity demonstrates how the brain does this, even if our eyes are seeing different things. By contrast, if we are only receiving information from one eye, our brains find it very difficult to judge distance and depth accurately, which in turn makes catching a ball a very tricky matter, indeed!
Hey, Catch!
This is an hilarious outdoors illusion. Get a friend and each make an eye patch with card and hat elastic. Head out to a nice open space (like an oval!), put on the eyepatch, and try to practise catching and throwing.
The inside story
An interesting fact here is that depth perception seems to be something we learn from experience. Anthropologists have documented cases where people who have lived all their lives in a jungle cannot identify animals seen far away. These people have never had to judge long distances by object size so they misinterpret this information. In one case people thought distant buffalo were actually ants.