Hold your hands outstretched in front of you, one twice as far from your eyes as the other...

Hold your hands outstretched in front of you, one twice as far from your eyes as the other, and make a casual judgment as to which hand looks bigger. Most people see them to be about the same size, while many see the nearer hand as slightly bigger. Almost no one, upon casual inspection, sees the nearer hand as four times as big. But, by the inverse-square law, the nearer hand should appear to be twice as tall and twice as wide and therefore seem to occupy four times as much of your visual field as the farther hand. Your belief that your hands are the same size is so strong that you likely overrule this visual information. Now, if you overlap your hands slightly and view them with one eye closed, you’ll see the nearer hand as clearly bigger. This raises an interesting question: What other illusions do you have that are not so easily checked?