The catchphrase of a new anti-drug commercial.
Transcript as follows:

A teenage boy is walking by an alley in an obvious "wrong side of the tracks" part of town. A group of tough guys come over to him.

Tough Guy #1: Hey man, how's it goin, hey, you wanna smoke some weed?
TG#2: Yeah, yeah, it's real good stuff.
Our Hero: Ah, no thank you, I.. uhh...
TG#3: What's the matter, man, you scared?
(Guys all start making chicken noises at him, and one of them starts pushing him, until he runs away sobbing.)

Cut to: Our Hero's house, he's running up the front walk, and into the door.

Voice over: Sometimes, the best advice on drugs come from your parents.

Cut to: Inside of house, dad sitting in rocking chair, smoking a pipe. OH runs in, and explains the events that transpired.

Our Hero's Father: Hmm.. well son, next time one of those tough kids asks you to smoke pot, tell them "MY GRASS IS FOR MOWING". (nods)

Our Hero: (smiles)(runs back outside)

Cut to: same alley, OH comes running up to the group of guys, stops right in front of them, plants his feet, and says:

OH: My grass is for MOWING!

Cut to: across the street, OH being chased by irate group of tough guys.

Cut to: black screen, with SFX of scuffle taking place.

Voice Over: Talk to your kids about drugs. They might even help other teens.

Cut back to one tough guy holding Our Hero's arms as another punches him in the chest repeatedly. Fade to black screen with the words "Campaign for a Drug-Free America".

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Warning: Nodeshell Challenge/Rescue