At 15 I was a normal teenage boy. That means I masturbated every chance I got. I discovered that if I just unzipped my pants and pulled my dick out, instead of opening them at the waist, it worked as sort of a cock-ring. At that time I had no idea what a cock-ring was. But I realized this technique gave me extra hard erections. The down side to this is, it's impossible to put it back in quickly.
So, there I was, rubbing my big stiff dick in my bedroom, when my younger sister just walked in!! She immediately looked at my stiff dick sticking out of my zipper, and I couldn't do anything about it. Her eyes opened wide, she sort of gasped, and I told her to get out. She just stood there staring. No way could I bend the thing to get it back in. So I just said 'you know boys do this. Do you want to watch?' She said 'gross', but then sat down right across from me.
My heart was pounding so fast, and every beat made my dick throb. So, I just sat there for a couple of minutes squeezing it at the base and wagging it back and forth. I knew if I started stroking, I would come instantly. Her eyes never left my cock for a second. I had pre-cum already dripping like a candle down onto my pants. I told her that I was going to come. I stroked it real slow knowing I wasn't going to last long. Then I said, 'I can't stop it...here it comes.'
I shot my load at least two feet in the air, and came more than I ever did before. My sister had a look of delight in her eyes. She enjoyed it as much as I did. She promised she wouldn't tell anyone, and as far as I know, she never did. I did it in front of her many times till I got serious with one of my girlfriends. We don't talk about it, but I'm sure she thinks about it as much as I do. Great memories.