What's real? What's not? Reality is strange-and seeing isn't believing. When you look at an optical illusion, your brain tries to fool your eyes and what you expect to find on the page isn't what's there.

Apart from being both endlessly fascinating and awe-inspiring visually, these illusions also force viewers to go outside their comfort zones, challenge the nature of perception, and think creatively. Their minds have to work overtime to become more flexible. That's what makes these 240 puzzles, ranging from simple scintillation effects to impossible staircases and concealed creatures, so absolutely addictive.

See how Santa "measures up" to his helper; the scale doesn't seem to be telling the truth! Compare two lines- one seems longer, but can't that really be true? Look at a triangular sculpture and figure out if it's "impossible." Move a picture from side to side and watch it spin. Try to "remove" a magic glass from the plate, count the number of "F"s in a line (but only read it once!), and locate the main subject in the masterpiece "Las Meninas," painted by the artist Diego Velazquez.

This collection of illusions will enable you to experience the impossible and marvel at the miraculous.