This duck/rabbit drawing is one example of an optical illusion. It was created by psychologist Joseph Jastrow to show that what we see is influenced by the things that we like or by our past experiences! So if you like ducks better than rabbits, you're more likely to see the duck first!
For more fun illusions that you can look at together, check out this website:
http://slodive.com/inspiration/optical-illusions-for-kids/