The pupil of your eye is the black circle in the center that lets light in. When you look in a mirror, the dark spot surrounded by your colored iris is your pupil. It's actually an opening, not a solid thing, which is why it looks black: you're seeing into the dark interior of your eye.

Your pupils constantly adjust their size. In bright sunlight, they shrink to tiny dots to protect your eyes from too much light. In a dark room, they expand wide to let in as much light as possible so you can see. This happens automatically, without you thinking about it. Doctors shine lights in patients' eyes to check if their pupils are working correctly.

The word pupil also means a student, especially a young one learning from a teacher. A piano teacher might have twenty pupils who come for lessons each week. In many countries, elementary school children are called pupils rather than students. The word suggests a closer teaching relationship than “student” does: a pupil learns directly from a specific teacher or mentor, while a student might be one of hundreds in a large school.