Our pupils change size according to the amount of light that surronds us.
More light: smaller pupil
Less light: bigger pupil
It changes size because to see, we need light.
When there is a lot of light the pupil can stay small, but even so, the light will reach our eyes.
When there is a decreased amount of light in our surroundings, the pupils increase size making the chances of capturing light higher.
Hope it helped,
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