In the figure, you can see that the iris, or colored part of your eye, is responsible for the opening we call the pupil. The iris is made up of two sets of muscles; one set causes your pupil to dilate (enlarge) to let in more light so you can see things better. This happens automatically when the level of illumination decreases and also when you’re extremely interested in something.

Imagine for a moment that you’re a car salesman talking to a customer who’s looking at a new car. Could you use what you’ve just learned about changes in pupil size to gauge the customer’s interest in the new vehicle? Instead of being the car salesman, suppose you’re the customer. How could you keep a car salesman from using this involuntary reaction of your pupils to know how you feel about the car you’re thinking of buying?