Optical illusions occur as a result of perception. Essentially, visual perception involves a three step process. 1) The eyes see 2)The information received is relayed to the brain 3)The mind reasons it all out.
Sometimes, though, what we see isn't what's really there. Below are three static, non-moving pictures from Mighty Optical Illusions. Take a look at how they "move" and "float" despite being fixed objects. This is a great lesson in learning how perception is not always an accurate experience. Gives new meaning to "seeing is believing", doesn't it?