I saw the "Nails in the Fence" story on Facebook, and used that as my lesson.
Nails In The Fence
Author Unknown
There once was a little boy who had a bad temper. His father gave him a bag of nails and told him that every time he lost his temper, he must hammer a nail into the back of the fence.
The first day the boy had driven 37 nails into the fence. Over the next few weeks, as he learned to control his anger, the number of nails hammered daily gradually dwindled down. He discovered it was easier to hold his temper than to drive those nails into the fence.
Finally the day came when the boy didn't lose his temper at all. He told his father about it and the father suggested that the boy now pull out one nail for each day that he was able to hold his temper. The days passed and the young boy was finally able to tell his father that all the nails were gone.
The father took his son by the hand and led him to the fence. He said, "You have done well, my son, but look at the holes in the fence. The fence will never be the same. When you say things in anger, they leave a scar just like this one. You can put a knife in a man and draw it out. It won't matter how many times you say I'm sorry, the wound is still there."
The little boy then understood how powerful his words were. He looked up at his father and said "I hope you can forgive me father for the holes I put in you."
"Of course I can," said the father.
I read the story, but left out the part about putting a knife in a man (a bit too gory for my taste).
Then I had each kid take turns hammering a nail into a piece of wood and then taking it out.
The kids enjoyed the hands on aspect of the lesson, and I thought this was a good way to demonstrate the damage that can be done when we say and do things in anger.