I can't believe people didn't notice this moment from "Beard Boys":
The two girls have just walked off after telling our heroes "find us in 10 years" (to which, of course, they reply "YESSS!")
Beavis strokes his "beard"...looks down at his hair-covered palm...and says, "Hey, Butt-Head, how'd those girls know we've been wacking?"
If you don't get it, like, ask your parents and stuff.
Huh huh huh...I said "ask".