A man called his two sons into the room one day, when they were about 14 and 15. He said to them, "Whatever you do, I don't want you to masturbate in the shower."
Well, the boys didn't know what to make of this and were too embarassed to ask their Dad why.
Many years later, after the boys were grown and had families of their own, the three of them met around a drink after a family dinner. One of the boys said, "Dad, do you remember years ago when you told us not to masturbate in the shower"?
The father replied, "Yes, I remember."
"Well," said one of the sons, "Why did you tell us not to masturbate in the shower?"
The father replied, "I didn't want you to use up all the hot water."