I was 12 when I heard this question, 'Let me see your thing.' My 15 year old neighbor asked it.
Jack was the son of the family living next to us. He was a rather big boy for his age, about 5 feet nine inches tall. I never paid a lot of attention to him in view of the differences in our ages, but unknown to me, he had been spying on me. Our family had a swimming pool in the rear of the house which was completely surrounded by a ten foot fence and many trees, so it was invisible from the outside or so we thought. It turned out that Jack climbed up in the trees after dark, and had a full view of the pool. When I was alone in the evenings, I would swim nude because that made me feel very horny, and I got a raging hardon in the process which I relieved with a good peter beating in my room after the swim. I did every thing other than coming in the pool, but didn't go that far because I didn't want to leave a ball of come floating in it.
I didn't know that Jack had been witness to several of my sessions, and became super hot over watching me walk naked with a huge hard penis swaying side to side as I left the pool to go to my room where I completed the job. As things turned out, I spent a week at Jack's house while my family was out of town, and I slept in his room in the other of the two beds in it. The first night I was there, Jack was restrained, but it was obvious to me that something was on his mind. I couldn't imagine what it was until on the second night after we had showered and were sitting in his room together out of the blue he asked, 'Let me see your thing.' I didn't know what to say, and just stared at him. He continued with,
'Don't be shy. I know you jack off. I've seen you doing it in the pool.'
'That's not possible,' I answered. 'The pool is behind a fence and all those trees.'
'Yes, I know. I climb up in them, and I know you jack off.'
The surprise was so great that almost in a trance, I slipped my now hard pecker out of my pjs. Jack looked at it appraisingly, and said,
'You've got a big one for your age.'
'Let me see yours,' I responded.
He got his out, and his remark proved accurate. Mine was just as large as his. Both of us were about five inches at the time. We must have done it together at least ten times before my family returned.