At least the first one I remember.
I think I was in 6th grade (though it could have been early in the 7th) which would have put me at 11 years old (or 12 years old if it was 7th grade). Anyway, I'd discovered that rubbing the underside of the base of my penis felt really good when I got hard but that was about as far as I took it, initially.
I came home from school one day and was hot and sweaty. I stripped out of my clothes and went and took a shower before sitting down at my new desk to do some homework. After getting out of the shower, I toweled off part way to get much of the water off me, got my underwear out of the chest of drawers, went to my bedroom (formerly the den), tossed my underwear onto my bed, and closed both doors (one to the kitchen and the other to the hallway). Sometimes I would lay naked on my bed feeling the coolness of the sheets as the rest of the water evaporated from my skin.
I didn't yet have a "real chair" for my desk, but I did have an antiqued wooden chair that we had recovered from our storage building on our farm when my dad sold the farm. It didn't have any padding for the seat. It was painted this semi-gloss antiqued light green color. On this day, for some reason, I didn't put my underwear and my clothes on. Rather, I pulled the chair up to the desk, got my books out and prepared to do my homework (naked). I must have been working on spelling/vocabulary because I remember that I had our Funk and Wagnalls dictionaries out (two-volumes. For those of you who are old enough "Look that up in your Funk and Wagnalls").
I would get these hormone induced raging hard-ons (though I didn't have one at this particular moment). But it was very nice sitting on this cool chair doing my homework naked. Sometime during this homework session I got the idea to take my penis, stretch it, and fold it and the scrotal sac under me so that the my penis head reach back towards (and made contact with) my anus. The feeling of these sensations coming from the underside of my penis, my perineum and my anus was "electric." I immediately started to get erect but with my penis folded under me in that way, the ability to "spring upward" was prevented. Rather I could feel my penis get warmer, larger, and longer. Very quickly the pre-cum started coming out of the tip of my penis helping to lubricate the chair's surface as I rocked back and forth. The "top" of the head of my penis was in contact with the chair seat surface as I slid slightly back and forth in a rocking motion. The underside of the tip of my penis was in contact with the outer anal sphincter muscles with some slight penetration. And finally, the "underside" of my penis was in contact with the perineum. All of this provided a very pleasant and intense set of sensations as I gently slid and rocked on the chair. I don't know how long I did this and enjoyed the sensations it was providing as it was a "fight" between releasing my penis and scrotum from under me and continuing the sensations I was experiencing.
I had been enjoying this sensation for some period of time when, all of a sudden, it started to get very, very intense. I liked it and kept going. The more I continued, the more intense and better it felt until, all of a sudden, there was this incredibly intense wave of sensations and a pulsation of the muscles in my pelvic floor and through the length of my folded penis. I became aware of some very warm liquid pulsing out of my penis "wetting" the chair and the area around the tip of my penis and anus. Things seemed suddenly more lubricated and I felt the "chills" and tingling through my body. I didn't know what happened in that moment except that I really liked it. As the sensation subsided and my penis began to get a bit flaccid, I finally stood up.
I looked at the chair and there was a small "puddle" of this somewhat thick white milky fluid on the seat. I was also between my butt cheeks and it had shot out the open back of the chair onto the rug and the edge of the bed behind the chair. I wiped up what was on the rug and my bed, but left what was on the chair and what was still dripping out of the tip of my penis. I knew "what" an orgasm was from the description. I just didn't know how it feel or how it would show up. As the milky fluid seemed to turn more clear and less viscous, I wondered if I could repeat this sequence to feel the sensations again.
I folded my penis and scrotum under me once again and sat on the puddle of liquid (now quite cool). This time there was a lot more lubrication as my folded under penis tried to get hard again. It took longer the second time and gave me a chance to extend the sensations. But the sudden rush at the end was just as intense. This time I sat back just a bit more as I reached orgasm to keep the liquid from shooting out the back of the chair. The tip of my penis was more squarely set against the outer muscles of my anus as each pulse shot out of me. I assumed that not much went into my rectum but it sure felt good.
After that day I started learning other ways to reach orgasm both faster and slower by rubbing myself. But that first orgasm gave "doing my homework" a completely different look.