Practice using the cross-eyed viewing method
Can you cross your eyes and make a third mouse in the middle of the other two who has both a tail and whiskers? If you do it right, the circles below the middle mouse should look three dimensional--the little circle in the center should look closer to you than the bigger circle on the outside.
Here's something fun! Cross your eyes and look at the colored circles below. Does it look like you're looking down a rainbow tunnel?
How it works: The slightly different angle of the circles gives the illusion of depth perception. In fact, this is how depth perception is created in the real world too! Our eyes are separated by a couple of inches, and our brains use this slight difference in angles to make depth and distance judgments!
Try this picture of Mickey Mouse for some more fun with depth perception! Cross your eyes to make a third Mickey in the middle. Mickey's head, hand, ear, and nose should look closer to you!
If you cross your eyes to make three Care Bears, the middle bear's tummy, nose, and right hand will be closer to you, but his left hand will be further away!
Can you get a third green elf in the middle? Does the picture have depth?
Can you make one clown in the middle by crossing your eyes? The clown should have a hat and a red nose. Is there a plus sign in the circles below the clown? If the hat, nose, or one of the lines in the plus sign tries to disappear, it means your visual system is working so hard that your brain is trying to shut off an eye. Blink a couple of times to see if the picture comes back.
Note to Parents: Children who have unstable eye teaming systems learn to suppress--in other words, their brain learns to protect them from double vision by blocking the image coming in from one eye. Suppression is not normal and is usually a red flag that the child's binocular system is in trouble. If you notice this happening, you might want to consider having your child's visual system evaluated by a developmental optometrist.