"You cannot change the state of a variable once it has been set"
http://www.onlamp.com/pub/a/onlamp/2007/09/13/introduction-t...
Im not a cool kid.
Im not a cool kid.
Here's an introduction if you've got an imperative/OO background: http://www.defmacro.org/ramblings/fp.html
Edit: Also, programmers in non-pure functional languages will often still minimize their use of mutation (changing variables) for a number of reasons.
If you think that immutable variables reduce programming errors, you are free to just not change your variables anyway, I guess. No language constraint required.
And actually, you could ignore functional programming and write code in an imperative style in Lisp (though it's not recommended, since you'd be missing out on the benefits of using Lisp).
Take a look at "Imperative Outside In" (chapter 3, section 2) in "On Lisp" (http://lib.store.yahoo.net/lib/paulgraham/onlisp.pdf) for an example.