Have you ever tried putting water and cooking oil in the same cup? They separate right away! No matter how much you stir, they don't mix. The Bible says our heart is just the same. If you're an upright child, who likes to tell the truth and follow the rules of the game, you're going to feel really bothered if a little friend starts cheating in the game or using bad words.
And you know what? The child who's doing the wrong thing won't like being near you either, because you do what's right and they want to make a mess! Jesus wants us to be like clean water. We don't need to fight, but we can't mix in with the ugly things.