I learned how to slide in my coaches back yard on a "Slip-N-Slide" around 1975. We learned the straight slide, hook slide, and the head first slide. When the coach wasn't looking we tried to do a pop up slide but we about killed ourselves trying. He wasn't very happy about that. We literally had sliding lessons every Saturday after our games that entire season. I found out later that the real reason we had Saturday sliding lessons was because our parents got together every Saturday after the game to grill out and consume adult beverages. The Slip-N-Slide was really nothing more than a babysitter, but everyone on the team did learn how to slide.
Len