What are everyone's favorite "aha" moments in learning Haskell?

I've been learning Haskell for the past couple of months, and as an avid programmer in other imperative languages, I'm always surprised at how random new concepts in Haskell can take a while to stick, even if I read about them and attempt to do some exercises, sometimes that isn't enough. So, it lead me to wonder what kind of "aha" moments others have had. Story time?