Why does my cat swing her tail when I give her a treat?

I give my kitty some soft butter on a spoon once and a while. While she is licking it off, she has this sweet, happy look on her face too. But, why does her tail swing back and forth?|||HAPPY^_^ !!!!|||She must be excited or content for getting such a rare treat. If it thrashes violently it means she's angry, but I doubt that's the case.|||It's simply a way of showing happiness and pleasure. Such as a dogs wag their tails. Same thing.|||Awww, she's just content with her treat. My cat rubs my leg whenever he gets wet food. She is communicating that she enjoys the food you are giving to her. Cats are cute!|||because she's enjoying her treat.|||she is waging her tail because she is happy just like dogs do|||cause shes happy.my cat does that when i give him nutella haha|||you made her happy