In Forth, you can do things like, say, redefine the number 0 to be computed as a function, and all code that uses the number 0 will instantly change its behaviour at runtime. Why would you do that? I’ve never found a legitimate use for it, which is why I hate Forth (and Lisp, for similar reasons). I like static analysis and I like it when the language prevents me from doing something silly, but I can understand why some people like the elegance and power-rush from one of the god-like languages like Lisp.