如何创建仅包含objective-c中的函数的文件？

I want to make a file that contains useful functions for my project. Well, I know that I can define a function inside my class implementation. But I don't really get it... I mean: I don't want to create a "class", just functions. Or are functions always part of a class? My objective-c knowledge is pretty limited, and my nice big book on objective-c does mention them, but does not explain how to implement them stand-alone.I want to make a file that contains useful func