One thing about Prolog is what I called 'backwards programming'. You write a function, say, that finds a route through a subway map from one named place to the named destination.

Once that's done, you can reverse the arguments to the function, and it can find a route backwards.

That's the best way I can describe it.

Basically, it's pretty cool and unique out of the languages I've tried, but you need a good teacher to get the most out of it.