Original Description:
Your eyes and brain can work together to make you see things that aren't there.
To see an example of this, first open this page of coloured squares, and try the following tests.
Eye Tests
Stare at the white cross in the middle of the squares for thirty seconds. Don't blink! What do you see when the colours change?
Repeat the first test, but this time hold your mouse button down on the cross when the colours change. That'll freeze the screen. Keep staring at the cross. What do you see?
Keep your eyes moving by following the white dot. Now what do you see when the colours change?