"But he was wagging his tail before he bit me!"
Many dog bites could be prevented if humans were better at reading dog body language. Contrary to popular belief, a wagging tail does not always mean a dog is happy. The position and speed of the tail wag, in addition to the dog's posture, ears, eyes and mouth can tell you volumes about whether the dog is happy, fearful or highly aroused.
Most animals are masters at reading body language. Humans also have this ability, but we tend not to utilize this skill because we rely so much on verbal communication.