Scientific American is doing a whole issue on optical illusions and they've got a sampling up on their website as a slideshow. It's interesting to see how some illusions are created by physical tricks of the eyes, while others are, apparently, psychological in nature. This illusion, for instance, is supposed to be influenced by the way sexual maturity changes your frame of mind—kids see dolphins and grown-ups see, well, sex.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.