A pupil is the opening in the center of the iris of the eye that allows light to enter. It appears as a dark circle and changes size depending on the amount of light in the environment. In bright light, the pupil constricts to limit light intake, while in dim light, it dilates to allow more light in. The size of the pupil can also be influenced by emotional responses and certain medications. For example, when a person is excited or scared, the pupils may dilate. This response is part of the body's fight-or-flight mechanism, helping to enhance vision in critical situations.