Do you see a duck or a rabbit?
When you first look at this picture you'll probably see a rabbit facing to the right.
However, if you continue to look at the picture it'll flip to become a duck looking to the left.
This is known as a 'bistable' image. You won't be able to see both pictures at the same time and
will instead flip between seeing the duck and then the rabbit. This illusion was originally
popularised in 1899 by American psychologist Joseph Jastrow, who used it to make the point
that we 'see' with our brains as well as our eyes.