According to some study, attractiveness and mood may trigger pupil dilation. The pupils control how much light enters the eyes. When the eyes require more light, they expand, and when the eyes want less light, they contract. The eyes play a significant role in social interactions. A person with large dilated pupils is seen as interested or excited by something. Pupil dilation can be caused by many factors. Physical activities that involve the muscles of the face and body, such as laughing, crying, frowning, and smiling, will tend to open up the pupils. So will mental activities that involve thought and emotion, such as worry, anger, fear, joy, surprise, and interest. Thinking about something attractive or appealing will also tend to expand the pupils. The opposite effect - contraction of the pupils - can be achieved by applying pressure to the eye balls. This is done to reduce the amount of light that reaches the retina, which could cause pain or blindness if it was not undone. People who do this for medical reasons include astronauts and soldiers operating night vision equipment. In addition to these two actions, pupils can be influenced by other factors such as darkness, size, color, and distance. For example, when looking at a bright object, such as the sun, the eyes will automatically constrict to prevent damage to the retina. This is because sunlight is strong enough to hurt your eyes directly.