Joke: Make Like A Frog-Fool joke

A seven year old boy goes to the hospital with his grandmother to visit his grandfather. 

When they arrive, Johnny runs ahead of his granny, and bursts into his grandfather's room. 

"Grandpa", the boy says, "As soon as Grandma comes into the room, make the sounds of a frog," 

"What for?" asked his Grandpa. 

"Grandma said that as soon as you croak, we're going to Disney World"