Why Can Cat's Pupils Change in Size?

Cat's pupils change three times a day to adapt to the changes of light. At noon, when the light is strongest, the pupils become a thin line. In the morning, when it is moderately light, the pupil is bigger, like a jujube-pit. At night, in complete darkness, the pupil becomes large and round and can see anything. Because of their special eyes, cats can catch mice in darkness.