There is a very good, recently discovered illusion which you can see if you follow the link on Shimojo's page to "Visual Illusion induced by sound." Dr. Shimojo demonstrated this one to our neurobiology class.
The setup is a screen with circles at the top-left and top-right corners. The circles both move to the center of the screen and continue to the opposite corners, so that there are two ambiguous interpretations; one is that the two objects passed by each other; the other is that they bounced off each other.
Now, most people will say that they saw the balls pass by each other without touching... but if there is a loud sound when the balls pass by each other, everyone will say that they saw the balls bounce off each other! The effect is quite striking, so I recommend you view the Quicktime movie for yourself.
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