When a mother demanded of her son why he had been smoking pot under the bridge with the other kids, he told her it was no big deal because all the kids do it.
“And if all the kids jumped off that bridge, would you do it too?” the mother asked.
“Oh, Mom,” the son replied, “Why do you say that stupid phrase every time I’m in trouble. It doesn’t even make sense.”
The mother thought for a moment and then said, “Because all the parents do it.”
See the whole collection at Power Shorts: The Shortest Stories.