During the summer vacation from school I would often go to my friend Jeff's house for the day. His family had a pool and it was great fun spending a few hours splashing around during the hottest part of the day. Occasionally his younger sister Jenny would join in. She was a couple of years younger than us and I just looked upon her as my friends bratty little sister.
One day, Jeff's mother came outside and asked Jeff if he would accompany her into town to collect some heavy bags of garden fertilizer. She said I could stay in the pool if I wanted, as they would be back in about an hour. Off they went leaving me and Jenny in the pool. Without Jeff to lark around with and Jenny just swimming up and down, I decided to leave the pool and get dressed. I went into their summer outhouse which had a shower and small changing area. I showered with my swimming trunks on (they were like Speedos) to try and wash some of the chlorine out of them, when Jenny marched in. I moved to the adjacent changing area whilst she got in the shower. I sat on the wooden bench watching Jenny showering as she had her back to me. I'd never looked twice at Jenny before, but her pert butt and shapely figure had me stirring in my Speedos. Her red bikini bottoms had begun to creep into her butt crack as she bent to wash her legs. She turned around facing me and it was then for the first time I realised what a nice pair of tits she had. They were full and bouncy in her bikini top, and each time she bent over I had a great view of them.
By now my cock had unfurled in my Speedos and had formed a long ridge pointing towards my stomach. I slung the towel across my lap hoping it would subside enough for me to leave the summer house. Jenny had finished showering and sat next to me drying herself. 'Got a boner?' she asked me. 'No' I replied, at which point Jenny whipped the towel from my lap and, much to my surprise, grabbed my cock through my Speedos. 'What's this then?' she asked as I sat there dumbfounded, '...feels like a hard cock to me'. She began rubbing my cock through my Speedos, the silky fabric sending tingles through my hard dick and into my groin. I was still speechless, never having seen or heard Jenny like this before. 'Bet I can make you cum in your shorts' she said as she took off her bikini top. Her little titties were nicely rounded with hard nipples as she continued to rub my cock through my swimming shorts. 'Go on, touch them if you want...I won't tell anyone' she said to me as I tentatively began to feel her breasts. 'Come on, cum for little Jenny' she whispered in my ear as her stroking got faster. By now I had both her titties in my hands, and I was gently pinching her nipples. 'Oh that feels good, is it good for you,' she asked, as her rubbing me through the silky material was beginning to make the head of my cock poke above the top of my Speedos.
It wasn't much longer before I could feel that sensation deep inside the groin and I knew I would cum. By now the head of my cock was free of my Speedos and I lay back against the wall as Jenny's hand was frantically moving up and down. 'I'm gonna cum' I breathed as several ropes of cum shot forth up my chest and stomach. 'Oh yes' replied Jenny as she had achieved her goal of making me cum. She milked my cock for a minute or two as I came down from my orgasm. 'You lost your bet' I said to her 'I didn't cum IN my shorts'. 'Oh well' she said with a wink, '..maybe next time!'