When the combination of color, shading, shape, light, and design cause the eye and brain to create a picture of something that is different than what it really is. This may include seeing colors that aren't there or are different, seeing curves in straight objects, thinking two things are different length when they aren't, or just plain seeing things that aren't there.

Examples:

• Flipping optical illusion
• Herring Illusion
• Impossible triangle
• Muller-Lyer Illusion
• Poggendorff Illusion
• Zollner Illusion

Is this an old hag or a young bride?

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credits: alt.ascii-art