If you want to keep your affection for someone secret, stay in dim light when he's nearby. "When you’re attracted to someone, your pupils tend to dilate," Dr. Hansra divulges. You can thank the sympathetic nervous system, which controls pupil dilation, for giving you away. This doesn't occur only when you're attracted to someone — you can also go wide-eyed when looking at anything aesthetically pleasing, from a tempting dessert to a stunning sunrise.