When I was a kid the thing to do was drop water bombs (balloons) down on people. Not on cars but you get the idea. Like you said dropping 10 inch rocks on cars never occurred to us.
Decades ago I played ball with a guy who was the master of bad luck. Once he was at a Lions game at Pontiac Stadium when someone in the upper walkway dropped a full container of beer straight down on his head (not poured), he got a concussion from it.
He was a car salesman and the following summer he was driving a new convertible representing the dealership in the annual 4th of July parade when a vintage tank that was behind him drove right up the back of his car before it could stop.....LOL!