An old farmer and his wife are going to town on their mule-drawn wagon. After a bit, the mule stops. The farmer can't get it to move, so he gets out goes up to the mule shakes his finger and says "that's one." A short while later the mule stops again. The farmer goes up to the mule, shakes his finger and says "that's two." When the mule stops for the third time, the farmer gets out shotgun in hand and shoots the mule.
As he is returning to the wagon his wife says "What did you do that for?"
The farmer looks at her and says "That's one."