请解释这个英语笑话?
A lady lost her handbag.It was found by an honest little boy and returned to her.
Looking in her purse,she commented,"Hmmm.That's funny.When I lost my bag there was a $20 bill in it.Now there are twenty $1 bills."
The boy quickly replied,"That's right,lady.The last time I found a lady's purse,she didn't have any change for a reward."