Most animals have tails - people and apes don't. A tail is just an extension of the animal's backbone. Animals use their tails to communicate - dogs wag their tails when they are happy, cats wave their tails when they are angry and put their tails straight up to say "hello". Animals like cows and horses use their tails to swat flies away. The kangaroo has a big, heavy tail that it uses to balance itself when it hops.