Years back, my mom had dropped me off at a friend's house for the day while both of our parents were at work. Since she had an older sister at home, my mom saw no problem with it.
I wasn't over there for long and we were outside when she started playing around and squirted me with the hose. I told her to stop but she did it again, so I fought the hose away from her and started spraying. It wasn't long before we were screaming and laughing having a water fight. When we were done and tried to go back inside, her sister got pissed at us and said we weren't coming in like that. We stood at the door and she went for a couple of bath towels, making us wear those. There is a part of her yard that no one can see, so she went first and I went after, stripping down and coming back to the house in just the towel. Her sister threw our clothes in the dryer and said when it was done we could put them back on.
We went up to her room to wait it out, listening to the radio and talking, but after a while she asked me if I ever played with myself. I told her no, and she said she wanted to show me. I went along with it, and she took off her towel and then mine, leaving us in our birthday suits for the first time. She spread my legs apart and rubbed her hands between my thighs, making me shiver. She took a finger and stuck it between my lips a little and wiggled it around, and I told her to stop because I was going to pee. She said I wasn't and kept going, and after she stuck her finger in about an inch and squeezed one of my nipples, I had my first orgasm making me buck on her bed and squeal. When I calmed down, she leaned back on the mattress and wanted me to do the same. I wasn't sure what to do, so she talked me through it. She was a little looser than I was, and I got my finger in further and moved it in and out, and soon moaned out loud and told me she was cumming. I didn't know what that meant, but she said it was what I just did.
She asked me if it was fun, and I told her it was, so we decided to do it again to ourselves. We sat on the bed side my side with our legs crossed over each other, rubbing ourselves furiously. We were both breathing heavily when all of a sudden her door opened and her sister walked in. She wasn't mad, but she said 'unless you're going to masturbate all day, your clothes are dry'. She then closed the door and went back downstairs. I was embarrassed but she wasn't, saying she watched her sister doing it without her knowing it before, so she knew it wasn't anything bad. We both started rubbing again, making ourselves cum before we went downstairs.
We were about to get our clothes out of the dryer when she asked me if I really wanted to get dressed. I said I didn't and neither did she, so we went out to the living room with her sister bare. She asked us why we were still naked, and we told her we didn't want to get dressed, and she just said whatever and kept watching TV. After a few minutes we started asking her questions about rubbing herself. She didn't want to talk about it at first, but she gave in, saying she does it every day and uses a vibrator. Talking about it was getting us horny again, and we asked if she wanted to do it, but she refused and wouldn't change her mind. We went on without her, sliding down on the couch and rubbing. She said we could do it there this time, but from now on to go to her room. We said OK and started doing it again, or 'masturbating' like she called it. We both came again, then just watched TV until her mom came back home. When the garage door opened we panicked and ran back to the laundry room as fast as our bare feet could go, getting dressed right before she came in.
That was all we did that day, but after that we would get naked all the time when we were alone.