"Cats are nothing but trouble." is like saying that there is not a thing good about cats. They are just troublesome. "Cats are anything but trouble." Is saying that they are easy to handle and are the opposite of trouble. Hope this answers your question...

Lol I don't think you would say nothing but... Because if you say nothing why would you need the but part.... For anything but... It like saying to a person they can get you anything except a particular thing because you don't like that particular thing.

Thank you Camille! I'm nothing but happy because finally I understand it! :)

"I'm anything but happy"- you are every emotion that is not happy. "Anything but" "I'm nothing but happy"- you are only happy, you could be nothing else. "Nothing but"

Those two example sentences you just wrote were perfect! Keep up the good work!