My botty's on fire

The whole family was around the rim of that amazingly big hole in America.
The Grand Canyon.
Went out for dinner, back to the car. Sam used to run with one arm pumping up and down to the side, like a really weird pirate with a dud parrot.
We all left the restaurant. Sam got into a bit of a run with his one arm pumping motion and ran into the back of a car and then sort of ricocheted off the bumper bar and then hit the ground and shot off like a bullet.
He slid for about 20 meters on his bottom,  got up and said....My botty's on fire.