Pupillary Response
Pupillary response is how your pupils grow bigger in dim light and shrink in bright light.
Think of it this way
A camera lens has a round opening that controls how much light gets in. In a dim room you twist it wide to let in more light. In bright sun you close it to a small circle so the image is not washed out. Your pupil is that same opening — it grows wide in dim light and shrinks to a tiny dot in bright light.
Explaining pupillary response by grade level
Look at your eyes in a mirror. The dark circle in the middle is your pupil. Turn off the lights and watch it get big. Turn the lights back on and it gets small. Your pupil changes size to let in the right amount of light.