What part of the eye adjusts in size to let more or less light enter?

Pupil

The Chihuahuan Desert mainly lies in which country? Which vegetable can be classified into types such as Cremini, Shiitake, and Portobello? Which popular 90s band had a hit titled "Semi-Charmed Life"? Which European explorer journeyed to China and served in Kublai Khan's court in the 13th century? Who owns the MLB record for most Grand Slams in a single season?