Making eye contact and grinning at a stranger can brighten both of your days. "Research shows it cheers people up," says Christine Carter, Ph.D., author of The Sweet Spot: How to Find Your Groove at Home and Work and a senior fellow at the University of California, Berkeley's Greater Good Science Center. "One theory about why is we're clannish, tribal people. Our nervous system feels safer in groups. So the more connected we feel to people around us, the safer and more relaxed we feel, and that opens up the possibility we'll be happy." Plus, grinning activates the smile muscles, which can also put you in a better mood.