I had been very good friends with the girl from my neighborhood, she was in a different class but in the same school and we usually met every afternoon at her or my place. Our friendship had started very early on and was changing slightly as we´ve been passing through puberty. This happened some years ago.
She has a three years older brother, so he was 16 at the time. He had his own room which had the ventilation system bordering with one in her room and when it was stopped in the upper position we could watch what he was doing. We did that quite a lot as kids are curious.
Then one day we saw him jerking off while on internet chat and we almost stopped breathing while standing at the table. We kept watching him for a good half hour and saw how he ejaculated in his hand. For both of us it was brand new information, even later that day she asked me whether I did that too, because she heard boys did this often. I denied it what was absolutely true at the moment and was surprised she knew such a thing. I was just at the beginning of puberty and would start to jerk off at my home several days or weeks later, after we watched him a couple more times. He did it daily at the same hour and it took him almost the same time to orgasm. Thanks to him I knew perfectly well how to do it.
Maybe a month later he had his birthday and he was given this really huge professional microscope. Several days later we caught him squirting his semen onto the Petri dish and examining it under the microscope lenses.
With boner in my pants and without thinking and figuring out the side effect of my thought I whispered I would like to see mine that way too. Immediately after I turned red and so she found out I was masturbating regularly too and was somehow angry knowing the reason why I ran home a little earlier every day during those last few weeks. I told her I masturbated late at night but she was right, I did it in the afternoon, sometimes even more than once.
The next day her brother had some exams so left home soon after we came in. After a while of doing usual stuff she went to his place, she made some huge noise there and brought the microscope with a Petri dish to her room. She told me she would take care that nobody would walk in on me and that I could watch my semen right now if I wanted. It was a great opportunity, I was very curious and when she promised to me she would not even look at me, I told myself I would do that. I don´t know why I didn´t go to the bathroom to do it secretly there, I just pulled my penis out of my pants and started to jerk off right then and there.
It was very thrilling knowing she was so close to me doing it in her room that soon I was pretty close to my orgasm. Then it happened-she asked me if I was already done, I couldn´t answer right after because I was so excited and so she walked in on me right at the moment of my orgasm. I didn´t hit the Petri dish, the semen was all around her table and everywhere. She started to apologize that it was a misunderstanding and so on. On the other side I felt great and I told her that it was the most amazing orgasm I ever had and would never hesitate to repeat it right after again.
She told me she would like to see me and that it was much more arousing than watching her brother. As I still had my dick in my hand and I was very aroused by this too I started to jerk off right in front of her. I asked her if she would join me but she only put her hand in her pants and touched herself a little. She told me that some other time she would show me but right now she wanted just to see me. I wasn´t very happy with that but horny enough not to stop doing my thing. That time I hit the Petri dish and then we watched my semen under the lenses. We didn´t see that much, the more interesting was the fact we did it together.
A little later we started to build the other kind of relationship, we stopped to watch her brother and rather we watched ourselves. We are still together so our sex life belongs to a completely different story, even sometimes we still watch each other while we masturbate and have fun reminding us how it started.