With both eyes open, hold two pencils about 12 inches (30 centimeters) in front of you, level with your eyes. Slowly bring the tips of the two pencils together. Not difficult? Try the same thing with only one eye open. You’ll find that it’s very difficult!

Each eye sees from a slightly different angle. Both eyes work together to give you depth perception—the ability to see things in three dimensions. You need depth perception for your brain to be able to judge distance. When you use just one eye, you have no depth perception.

Text by Julie Vosburgh Agnone
Art by Gregg Valley