Replacing my twin for me. Without her knowing.
One time when I was 15, me and my twin bro went to a younger guys house after school. Kind of waiting for our parents to finish work.
I was playing the computer for a while and wondered where my twin was. He was away for ages. I went to the room next door and tried to open the door. It was jammed with a chair. I managed to get a view through the opening briefly. I saw my twin on a bed with his trousers down and another girl a few years younger, grabbing his balls. We all went to the same school as we all wore the same uniform.
When he left the room after I tried to barge in. I went back into the room and pretended to her that I was him. So I jumped onto the bed with a full on hard on and waited for her to squeeze my balls.
She was suspicious at first but I pulled the trick off.
She then asked why my cock sticks up at a 45 degree angle.
Shortly after. I had to leave.
But I always wondered what would happen if I told her the real reason it was hard.
But that memory is always a good one. I should have asked her to jerk me off. But for some reason she just wanted to grab my balls.