The duck-rabbit drawing was first used by American psychologist Joseph Jastrow in 1899 to make the point that perception is not only what one sees but also a mental activity.

The above image originated in the 1880s as a tricky puzzle and was designed for children. In this optical illusion, there is a man's face hidden inside the rabbit’s sketch.

Is this a rabbit or a duck? The answer you give to this classic optical illusion says a lot about your brain, psychologists claim. It first appeared in a German magazine in 1892 under the title ...