A person's pupils expand and contract depending upon what they are thinking and how engaged they are in a conversation. When a person's pupils are large and remain large, it can indicate they are interested and engaged in what you are saying. Conversely, when the pupils get smaller, the person has lost interest, their brain is overloaded or their mind may be wandering. But don't stare at your prospects pupils too long. Looking into someone's eyes for longer than 5 seconds will make them feel uncomfortable.