When I was younger I would always fantasize about catching my sister masturbating. She was incredibly sexy for her age. The best I could do was occasionally pleasure myself with her panties. She was about 13 or 14 and just starting to wear sexier kinds of underwear making my jack off sessions with satin panties and other sexier items even more fun. I would always be very careful never to leave cum in the panties though.
One day I was finally able to catch her masturbating. She was in the hot tub and I assume she thought no one was home. I saw her in the hot tub through a window and didn't think much of it until I looked closer. She had on a sexy two-piece bathing suit and she was on her knees kind of leaning over the side of the hot tub with her back to me and her hips pressed against a hot tub jet.
I knew it was a jet she was pressed against because I would occasionally put my cock against the jet to get off.
Well, I noticed that her suit bottoms were riding up on her butt quite noticeably. She was clearly getting off and I was instantly hard. With no one else home I ran down to her room and grabbed the first pair of her panties I could find. I ran back to the window, took out my hard cock, and wrapped those panties around it and started furiously masturbating.
After a short while she started gyrating against the jet getting closer to orgasm. Her head would occasionally arch back or down as she got into it. The scene was bringing me close to orgasm, but I didn't want to cum before the show was over, so I tried as best I could to time my orgasm to when I think hers would be.
Finally I saw her push herself harder than ever into the jet and her butt cheeks clenched tightly. I knew she was about to explode. Needless to say as she was obviously cumming in the hot tub I had one of the most satisfying orgasms of my life at that time and filled her panties full of cum. I felt instantly bad about messing her panties and hid them in the trash.
It's a fond memory.