What is fun?

What exactly is fun?  A few months ago it crossed my mind that I really don’t know what it is. The question keeps coming up and I am still not sure. I clearly distinguish it from enjoyment because enjoyment often comes about from a release of tension, or stress, or from a chat with a good friend. Fun cannot be the absence of misery; it must something on its own. It’s not enjoyment and it is not contentment, but what is it?! I know I don’t have very much of it or I would probably be able to figure it out.