The tooth fairy

Conversation with O
O:  Let's play a game.  You be the mama, and I'll be the tooth fairy.  I will fly around and give everybody gifts.
Me:  What's a tooth fairy?
O:  A tooth fairy is someone who gives people gifts.
Me:  When does she give gifts?
O:  At night.
Me:  She gives everyone gifts every night?  That's a lot of gifts.  I bet she gets tired.
O:  Yeah, but she's a fairy.
Me:  Why is she called a tooth fairy?
O:  Cause she can fly.