Your smile makes other people happy, which in turn makes you happy.

If you could do something that would make people happy, and it would cost you neither money nor time, would you do it? If that same thing also made you happy, would you do it? What is this magical thing that will brighten your day and the days of people around you and yet cost you nothing? A smile.

Scientists at the University of California in San Francisco have identified nineteen different kinds of smiles, each of them capable of communicating a pleasant message that will often be met with a smile in return.

In a study of adults of various ages, a tendency was found for subjects to mimic the expressions of those around them. In other words, sad faces evoked more sad faces, and smiling faces evoked smiles and happiness. ( Lundqvist and Dimberg 1995)

Excerpt from 100 Simple Secrets of Happy People by David Niven, Ph.D.