Hold your hands outstretched in front of you, one twice as far from your eyes as the other, and make a casual judgment about which hand looks bigger. Most people see them as about the same size, although 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 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 (that Frank Oppenheimer posed to me years ago when I first taught at the Exploratorium): "What other illusions do you have that are not so easily checked?"