The one thing that a math problem, an orgasm, and a photo of a dead body have in common is that it can cause your pupils to expand. This dilation is one way that your body responds to various stimuli. It is very slight and uncontrollable. Scientists have known for over a century that pupils react to changing lights but are unsure why it happens when you’re exposed to any kind of psychological stimuli. Some researchers believe one reason may be that our brains are trying to send out messages.