Cats often use signs to tell the world what they feel and understanding this body language can go a long way to understanding them. One early sign of being vexed or cross is the swishing of the tail. When this escalates to smacking the tail on the ground or from side to side, this is a big red warning light - angry cat present! They will even smack the tail off furniture or your leg as a way of demonstrating their less than satisfied state. This is the opposite to the reaction in a dog, with whom a wagging tail is a sign of happiness.