I used to play Teachers (Freud would have a field day how we came up with that take on 'Doctors') with a friend who was a year younger. We'd ask each other questions and have to pay a forfeit if we couldn't answer-mostly I'd end up jacking him or feeling his erection. He was too young at that stage to cum-not that either of us knew it was possible. I'd end up highly aroused but frustrated, not knowing quite what was happening. I'm not sure how old we were but neither of us had gone through the hairy stage of puberty.
Anyway I was lying in the bath one evening tugging back my foreskin (he'd complained it was too tight and that I should be able to pull the skin back like he could) and using the soap to lubricate the glans. I remember feeling a very strange sensation, weird but extremely enjoyable. Suddenly I threw caution to the wind and rubbed my bare glans briskly between soapy palms. Before I knew it I'd shot several thick ropes onto my stomach.
It terrified the life out of me. For a moment I believed I'd damaged myself. Still, a few days later I repeated the technique and for a while it was my favourite method of masturbating.