Writing functions in functions.php

I’m beginning my foray into writing my own php functions and I’ve basically got it down, but when I try and add the functions into the functions.php file instead of directly into a template, they often don’t work.

For example, I tried creating this function:

function cn_parentcat ( ) { if (is_category('8')) { echo ('fubar');} }

If I add the if statement directly to a template (category.php in my case), a-okay. But it’s a no-go in functions.php.

Is there some query (or something else) I need to pass before I can perform certain actions like this?

Thanks!