You may be familiar with a 19th-century optical illusion — or, more precisely, “ambiguous image” — of a rabbit that looks like a duck that looks like a rabbit. First published in 1892 by a German humor magazine, the figure was made popular after the philosopher Ludwig Wittgenstein used it to illustrate two different ways of seeing. You can interpret the image as either a duck or a rabbit, but not both animals at the same time.
It gets trickier if you place two copies of the illusion side by side. You’re likely to see two ducks. Or perhaps two rabbits. In fact, about half of people can’t see a rabbit and a duck at first glance, according to Kyle Mathewson, a neuroscientist at the University of Alberta, in Canada. To picture one of each species simultaneously, you have to give your brain more information to work with—for example, telling yourself to imagine a duck eating a rabbit.