Iris and Pupil
Iris and Pupil is the pair of eye parts that control how much light enters your eye.
Think of it this way
A round tray with a hole in the center shows how the iris and pupil work. The tray is the iris, a colored ring, and the hole is the pupil. In bright light the ring spreads wide and the hole shrinks small. In dim light the ring pulls back and the hole opens to let more light in.
Explaining iris and pupil by grade level
Look at a friend's eyes in a bright room. The dark circle in the center is small. Now turn the lights down low. That dark circle grows bigger to let in more light so you can see.