A Different Kind of Jet Pack

My nine-year-old son was playing with the garden hose, again.  I watched as he sprayed a strong jet of water straight down, onto the driveway.

"Bubba?" I asked.  "What are you doing?  If you're going to use the water, at least water the grass or the plants."

"I'm making a jet pack," he answered.

"A jet pack?" I looked at that kid sideways.

"Yeah, Mama.  Watch how it works."  He sprayed the water onto the ground again and jumped up as he did it. "See?  It launched me into the air."

"Dude," I said.  "I think you launched yourself into the air!"

He shook his head.  "I did jump.  But, the the water made me go higher and stay up longer.  You try it."

I took the hose, sprayed the water and jumped. The only thing that was different from a regular jump, was that I got wetter.  "I don't think your hypothesis is correct."

He gave me a very serious look.  "Yes, it is.  You're just too heavy!"