When you look through the tube with one eye, and place your hand in front of the other eye you see a hole in your hand.

This is because your brain can see two different things with each eye: your hand and whatever is at the end of the tube. It mixes those two images together to create a picture of the world that makes sense of both things: a hand with a hole in it.

We need two eyes for seeing in 3D, or depth perception: how we work out how far away things are. When we look at objects, each eye sees something slightly different, because of the distance between the two eyes.

You can test this by looking at an object and shutting one eye, then the other. You’ll see that the object looks slightly different with each eye, you might be able to see more of one side of it, or it might move relative to the background. Try it with something close to your eyes, and something far away. There is more difference between the views from each eye when looking at the near object, than the far object. Your brain uses this information to know how far away things are.

It is very rare in real life to see something completely different in each eye, because most things are not that close to our faces. Look at something close to your face with both eyes open. You’ll see a ghost image where something is in front of one eye but not the other, for example if you lie on your side you’ll see a ghost image of the pillow, because the pillow is over one eye but not the other. You’ll also see two ghost images of some objects, because one eye’s view is so different from the other. You can get rid of the ghost or double images by really focusing on the object, but this can hurt your eyes.

None of these tests will work if you only have one eye. People with only one eye cannot see in 3D and have to use other cues to work out how far away things are.