"I learned it by watching you!" is the famous line from this 1980s anti-drug PSA produced by the Partnership for a Drug-Free America, a consortium of alcohol, cigarette, and pharmaceutical companies who didn't like weed taking a cut from its profits.
In the video, a boy's father approaches him with a box of marijuana. He sternly tells his son that his mother found it in the son's closet, and asks him where it came from and "who taught you how to do this stuff?"
The son then bursts out "I learned it from watching you!" The video dramatically ends as a narrator says "parents who use drugs have children who use drugs."
I think the mom may have been secretly watching from behind the door to make sure her husband scolded the boy, but as soon as she walked away, the father and son shared a nice joint together.
The PSA also inspired this commercial for Kraft Mac and Cheese: