One morning, Sally was riding to work. She ran over a big rock, and she fell off her bike. When she got up, her bike tire was flat. Then it started to rain. Sally got wet. She came to work in dripping wet clothes and wet hair. What's more, she arrived late. She felt sad all day. But after work, she went to choir practice. She sang lots of songs. And the singing made her happier. 

What do scientists say about Sally's experience? Grenville Hancox is a music expert. He knows how to play the c...