Jewish rabbis tell a poignant story that drives home the point of Proverbs 18:21. As the story goes (and five versions of this appear in Greek literature), Rabbi Simeon ben Gamaliel one day asked his servant to go to buy some good food for him in the market. When the servant returned home, he presented the rabbi with a tongue.
The next day, the rabbi told the servant to go to the market to buy some bad food. Again, the servant returned with a tongue.
When the rabbi asked the servant why he returned with a tongue both times, the servant made this astute observation: "Good comes from it and bad comes from it. When the tongue is good, there is nothing better, and when it is bad, there is nothing worse."
Sticks & Stones: The Discipleship of Our Speech, by William R. Baker