What has teeth but can't bite?

There were some backwoods hillbillies living across the river from each other, who feuded constantly. John hated Clarence with a passion and never passed up a chance to throw rocks across the river at Clarence. This went on for years until one day the Corps of Engineers came to build a bridge across that river. John was elated; he told his wife that finally he was going to get the chance to cross over and whip Clarence.
He left the house and returned in a matter of minutes. His wife asked what was wrong, didn't he intend to go over the bridge and whip Clarence? He replied that he never had really seen Clarence up close and didn't realize his size until he started over the bridge and saw the sign: "CLEARANCE 8 FT 3 IN"