Illusion #171 : Click the image and stare at the cross for 30 seconds - you might be able to make one of the letter 'a's grow.
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.