I was really pissed at my brother for teasing our younger sister. One weekend morning we got him back. We slipped into his bedroom while he was asleep and put his hand into a large glass of warm water. Then we ducked into his closet to wait.
Pretty soon he peed his bed and woke up. He was very perplexed but decided to clean up. He stripped completely, removed his sheets and blankets and then stopped because his dick started growing. He laid on his bed and jerked off onto his belly. Then he grabbed clothes in his hands and ran across the hall into the bathroom. Only then did we slip out.
This was in the 60's and my first view of a erect penis. We both enjoyed it very much and we were quite a bit nicer to him afterwards but neither one of us has ever told him.