When I was a kid, we had a sled run in our neighborhood that we called the ski run because it would descend about 75 yards and then level off for 5 yards before the really steep hill of about 100 yards.
When I was about 9, I suddenly noticed a tingling sensation in my crutch as I was riding the sled. The more I pressed my cock into the sled, the more it tingled and after several minutes, I would feel really good then have a warm wet sensation in my pants. I would do this four or five times and when I got home, I was amazed to see all of the cum in my pants, of course I didn't know what it was but I kept doing it every day.
One day we rode double with a cute girl riding on the bottom of the sled and me on top of her. When I started to tingle a little, I began to pump her and she said that it felt good for her too. Later on, we learned that we could do a lot more and we did, I would rub her pussy and she would stroke my cock and we would cum together. It was the best sex ever.