Pupil

Your eye has a little black hole in the middle of it called a pupil. Have you ever seen a black marker? When you color with it, it covers everything up and you can't see anything where the black is. That's how your pupil works too. When it gets bigger, it covers more of your eye and makes it darker. When it gets smaller, you can see more of your eye and it gets brighter. Your pupil gets bigger and smaller automatically, like when it's dark outside it gets bigger to let more light in, and when it's bright it gets smaller so you don't get too much light in your eye. Cool, huh?