Why do our pupils change size?

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In the middle of the iris is a round opening called the pupil, which looks like a black circle. The size of the pupil regulates the amount of light that enters the eye. Two muscles in the iris automatically adjust the size of the pupil to the level of the light. In dim light, the dilator muscle enlarges the pupil, so as much light as possible can then enter the eye. In bright light, the sphincter muscle makes the pupil smaller, which prevents too much light from entering the eye. The pupil also becomes smaller when the eye looks at the nearby object, thus bringing the image into sharp focus. From the book Tell Me About?