As you're all no doubt aware, the word "pupil" can mean "a schoolchild, or a person who is taught by another" and also "the dark circular opening in the centre of the iris of the eye".
You may not know that the two meanings are related. Here's the OED:
"The two [meanings] are related to each other, and to "poppet", "puppet" and "pupa", through their Latin root, "pupa" meaning a girl or a doll. Pupil entered English via Old French from the Latin forms 'pupillus' (diminutive of 'pupus', a boy) and 'pupilla' (diminutive of 'pupa', a girl). It originally meant "orphan or ward", and did not take on its modern meaning until the 16th century. Pupil meaning the centre of the eye is from the feminine form 'pupilla' (literally, "little doll" or "young girl"): it acquired its English meaning from the phenomenon whereby one can see a tiny reflected image of oneself in another person's eyes.