Old Or Young Lady?
Illusion #111 : Old lady or young lady? From a 1888 German postcard.
There are different types of illusion. Physical illusions are those such as a straw that appears to bend when placed in water. Physiological illusions can be caused by excessive brightness, colour or movement. Cognitive illusions are those that seem to contradict our logic, and even knowing that it's a trick doesn't help.