Here are two fun experiments that explore how our brain can sometimes be fooled by what our eyes see.
Take two pencils and hold one in each hand. Hold them horizontally, so that the ends are together, and bring them level with your eyes. The pencils need to be a few centimetres away from your eyes. Now look at the point where the two pencils touch. Carefully move the pencils apart. You should see a flying saucer!
If you don't have two pencils you can repeat this experiment using your two index fingers. Place them together end on end, and hold them in front of your eyes. Move them slightly apart and you will see a flying sausage!
For the second experiment, take an A4 piece of paper and roll it into a long tube a couple of centimetres wide. Secure it with tape. Hold this tube to your eye - it doesn't matter which eye, but you need to keep both eyes open during the experiment.
If you chose your right eye, take your left hand and turn it so the palm faces upwards. Raise this palm alongside the tube. You should see a hole in your hand. As you move your hand the position of the hole will also move.
Moving the tube up and down will also cause the position of the hole to change.
Your brain combines the images made by your left and right eye. When these images are very different, the brain interprets the images incorrectly.
Repeat the first experiment again, however this time look at the pencils first with your right eye and then with your left eye. The point at which the pencils meet together will appear to move from the left to the right, as each of your eyes sees a slightly different image. When the brain combines these two images it interprets them as two pencils and a flying saucer.
In the second experiment, one eye sees your palm and the other sees the hole created by the tube. Your brain combines these images and interprets them as a hole in the hand.