Smithsonian Magazine has an interesting article on how the Slip 'N Slide at-home water slide became an American institution. It started with some bored kids on a hot day:
It was the summer of 1960, and Robert Carrier had returned home from work to find his 10-year-old son, Mike, and his friends careening down their driveway in Lakewood, California. To escape the heat, the boys had turned the hose on the painted concrete, creating a cool, slippery surface to play on.
The next day, Carrier, who worked as an upholsterer with a boat-manufacturing company, brought home a 50-foot roll of Naugahyde, which he unraveled on the driveway. The vinyl-coated, waterproof fabric was a vast improvement—slicker and safer—over the perilous concrete. But Carrier's son had given him an idea.
The fine folks at Wham-O soon got wind of the wet wonder, and a sale was made. And the rest is history. Now we have YouTube bros seeing who can make the longest one or add a loop.
The Accidental Invention of the Slip 'N Slide (Smithsonian)