Just a heads-up, the following is kinda a NSFW-story, so don't tell me I didn't warn you:

When I was a kid (really young but not sure anymore of my age back then), my niece of the same age often came over, and as most of you, we had a normal childhood, playing games, having fun, so it wasn't weird that we already saw each other naked when playing in the kiddypool. As kids you have no clue what sex is and don't wonder about it either, but you always ask questions when you see new things.

So one day, after playing in the kiddypool again, I asked my dad why I and my niece looked so different down there. My dad, trying to give it a fun twist, said that I had an 11th finger and my niece a 2nd mouth. Believing my dad I was glad about the answer and went back to my niece to tell her what I learned.

Meanwhile my parents were still inside the house, my dad having a good laugh with my mom when he told her what he said to me. A little later, they came back to me and my niece to get us for dinner, only to find us in a weird position, what looked like we were trying to have sex. Both shocked, my mother screamed and yelled what the hell we were doing.. I answered that I told my niece what dad told me, and that we wondered if her second mouth had any teeth, so she said I should try and feel with my 11th finger.. It was at a later age we both realized what happened, and never spoke to each other again.

;Tldr: nsfw, when I was a kid, I accidently might have had sex with my niece without both of us knowing.