Why do you have to put your tooth under your pillow when you lose your tooth?

Traditionally that is where the Tooth Fairy has you put it, so that she can pick it up in the night. She is to leave a special surprise in the place of the tooth. In olden times, this was a penny, then a nickel, dime and quarter. I believe it's a dollar now.

Ask your parents, they'll have more up-to-date information on that.