Cats talk with their tails so if you can decipher their tail positions, you can get a pretty good idea what your cat may be thinking. Unlike dogs, cats usually have a specific reason to wag their tails.
If a cat watches a bird or rodent, its tail will often twitch with anticipation. If the cat later misses the bird or rodent, then the tail will often twitch in frustration. Now if people would only just give cats everything they want all the time, there would be no need to study a cat’s tail because the cat would just be happy all the time.