I read the post today about the fun bike ride, and it reminded me of one of my experiences, also when I was 13. I was in Boy Scouts, and one of our favorite activities was going on bike hikes on the C&O Canal, which ran alongside the Potomac River in Maryland.
Except when we came near tourist places like Great Falls, we would often go for a long time without seeing anyone. There were usually between 4 and 8 of us. Sometimes we'd ride close and talk, but other times we'd string out so we were pretty far apart. This one day I had been horney, and popped a boner while riding along. The path was fairly bumpy, so the vibration got me going. I rode with one hand, and with the other I held my penis down on the seat, enjoying the vibrations. As I rode, I dropped back behind the other guys, and was really getting into it. I could tell I was going to come, so I slipped my shorts up and pulled my penis out of the leg of my underwear, and massaged the head. There I was riding along in a near-orgasmic state, and then it hit me. I started to come, with four or five jets of semen sloshing out onto my thigh and dripping down my leg. It felt really good, but boy, was it a mess. One of the guys up ahead stopped, so I had to quickly stop or he'd see the mess on my leg. So I stopped and got off and headed to the bushes. It looked like I was going to take a pee, but I took that opportunity to clean up. I had to wipe off with leaves, but it worked OK (I had to be careful what leaves to use -- the year before I had to poop in the woods, and my friend Billy got me leaves which turned out to be poison ivy. I was very, very uncomfortable for two weeks after that, and never again trusted anyone else to get leaves for me!).
Anyway, that was a memorable bike ride. I don't think I ever did it again while riding, but I enjoyed that one time.