I used this illustration with pretty good results in my youth group.

Tell all the youth to say “Milk.”
Tell them to say “Milk milk milk.”
Tell them to spell M-I-L-K
Then say, “What do cows drink?”

Most of the kids will say “milk,” but the answer is “water.”

This really illustrates that our minds are easily shaped by outside forces. If we listen to unacceptable music, our conversation is going to start sounding like the lyrics. If we hang out with crude people, we’re going to start saying crude things. We need to spend time with God every day so that we can sound like him when we talk.