Kevin Tollefson, age 14, of Beaverton, Ore., for his question:

WHAT IS AN OPTICAL ILLUSION?

Now you see it, and now you don't. The magician is a master of optical illusion: He can make you think you see something that you don't actually see.

An optical illusion depends on a wrong emphasis upon cues or on misapplied habits  as in the tendency to overestimate vertical as compared with horizontal lines. Your eye is actually playing a trick on you  it just appears that a dozen three inch vertical lines drawn next to each other on a sheet of paper are longer than a dozen three inch horizontal lines sketched next to the first set of lines.

An optical illusion can be a false sense perception and often occurs when a person is in a state of emotional expectancy or preoccupied in thought.