This actually dates back to 1899, an American psychologist Joseph Jastrow developed the optical illusion to show perception and seeing has to do with the brain, as well as the eyes.
It's also being used today to test creativity in kids. 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.
They say if your kids are able to see both images, don't be surprised if you catch them daydreaming or making unusual connections between seemingly unrelated ideas—they probably see the world in a different way.
What do you see? A rabbit, a duck? Or both?