A little boy liked chasing little girls on the playground. They put a stop to that with 1 word.
It might seem small, but teaching kids from a young age the importance of stopping when someone says "stop" means they're more likely to respect each others' boundaries as adults too.
The whole conversation actually sounded like this, but I couldn't get it into 140 characters:
ME: "What did you do at school today?"
ME: "With who?"
HIM: "There are some girls I like. I used to to chase them, but they told me they did not like being chased. So I stopped. Now we all play what we all want to play."
ME: "That's cool little man. It's totally cool."