What is the missing number?

A little boy and his grandfather are raking leaves in the yard. The little boy finds an earthworm trying to get back into its hole. He says, "Grandpa, I bet I can put that worm back in that hole." The grandfather replies, "I'll bet you five dollars you can't. It's too wiggly and limp to put back in that little hole."
The little boy runs into the house and comes back out with a can of hairspray. He sprays the worm until it is straight and stiff as a board. Then he puts the worm back into the hole.
The grandfather hands the little boy five dollars, grabs the hairspray, and runs into the house. Thirty minutes later the grandfather comes back out and hands the little boy another five dollars. The little boy says,
"Grandpa, you already gave me five dollars."
The grandfather replies, "I know. That's from your grandma