From a reasonably common programming language, which do you find to be the most mind-bending?

I have been listening to a lot of programming podcasts and taking some time to learn some new languages that are being considered upcoming, and important. I'm not necessarily talking about BrainFuck, but which language would you consider to be one that challenges the common programming paradigms?

For me, I did some functional and logic (ex. Prolog) programming in the 90s, so can't say that I find anything special there.

I am far from being an expert in it, but even today the most mind-bending programming language for me is Perl. Not because "Hello World" is hard to implement but rather there is so much lexical flexibility that some of the hardest solutions can be decomposed so poetically that I have to walk outside away from my terminal to clear my head. I'm not saying I'd likely sell a commercial software implementation, just that there is a distinct reason Perl is so (in)famous. Just look at the basic list of books on it.

So, what is your mind-bending language that promotes your better programming and practices?