Thinking about dark places can be enough to dilate your pupils on command. Staring at something in the distance or letting your eyes fall relaxed and unfocused makes your pupils grow. Other methods for dilating your pupils include tensing your stomach or imagining yourself in a daring situation.
Another way to dilate your pupils is to ingest caffeine, such as in a strong coffee drink or by taking a caffeine pill. The pupils tend to dilate when people are in dim light, are full of adrenaline or see someone who is attractive. Dilating your pupils on command can also affect how others perceive you.