The Muller-Lyer Illusion
What to see and do: As shown below, the two twin sisters are smiling, and the only way people in town can tell them apart is the "different" length of their smiles. Actually, the smiles are the same length. If you don't believe me, drag the smile on the left to compare it to the other smile. *Hint* when clicking the smile, try clicking the bottom of it.
What's happening: The creases of their smiles make the "lips" seem different lengths. One crease makes one smile look smaller while the other crease makes the other smile look bigger.
Credit: This illusion was created by a German psychiatrist, Franz Muller-Lyer in 1889.