When I was thirteen, like all boys, my hormones were running high. My parents were divorced so it was just mom and me. During the summer, she was at work and I had the house to myself, and I would enjoy walking around nude. Of course, that always gave me a really good erection, and I would masturbate myself to a healthy climax.
One day I was feeling particularly turned on, and my erection was super hard. I was walking around the house, having fun with my penis sticking up like that, and savouring how rigid it was. When I was walking through the living room, to my complete surprise, I suddenly ejaculated. The liquid just came squirting out, and I couldn't help it. I had never had it just happen like that before. It was the most fantastic thing, and it practically took my breath away!
Well, after that time, I began to enjoy trying to get my penis as absolutely rigid as possible. I would keep it as hard as possible and for as long as possible in a kind of torturous pleasure, not touching it. Then after a while, it got to the point where I would spontaneously ejaculate. I loved it, and got pretty good at doing this for a while.