What's an uncle?

[freyley]

You had, when you were growing up, relatives. I'm fairly sure of it, at least, I've been told you did. Some of them, you liked. Some of them, meh. There were, of course, the terrible ones worthy of telling stories, but in some ways, those are good relatives too.

Parents, grandparents, even cousins, have relatively well-defined roles in my head.

But uncles? What, pray tell, was your best avuncular experience? What makes a good uncle?