This optical illusion was used in 1899 by American psychologist Joseph Jastrow to show that perception and seeing also has to do with the brain, instead of just the eyes.
If the participant sees both images and can switch between the two images easily, Jastrow proposes it's an indicator of a person's creative abilities and how fast his or her brain works. During testing, those who can see both images were able to come up with five unusual uses for an everyday item; meanwhile, those who had trouble switching between the two could only come up with two unusual uses for the same item. So if your kids can see both a rabbit and a duck, don't be surprised if you find them daydreaming often or making unusual connections between seemingly unrelated ideas—they probably see the world in a different way.
When testing children at different times of the year though, the results can (understandably) change. During Easter, kids are more likely to see a rabbit first, while in October, they usually see a duck first.