The black part of the eye through which light enters, it enlarges in dim light and decreases in size in bright light.

Changes to pupil size can also be triggered by emotional responses to stimuli, which can be monitored and interpretted as subtle visual clues to how one person reacts to another.

Mind or body altering substances/intoxicants can cause a pupil's reaction to light to differ from that of a non-intoxicated individual, with varied different abnormal responses, which also can be monitored and interpretted as visual clues of what a person is under the influence of.