Grandy's Grand Inventions

Judy Cameron adores her grandfather, but there's a problem. Grandy loves to invent things, and his inventions rarely turn out well. Instead, they always seem to make a terrible mess throughout the house. For that reason, Judy's mom does not welcome Grandy's visits. Judy has been promised any one thing she wants (within reason, of course) if she earns straight As on her next report card. She does, and furthermore her birthday is coming very soon. How can her parents deny her wish-a visit from Grandy-even if he brings along another weird and wacky invention?