Static methods vs. plain functions

Hi,

I am working on an OOP project, and cannot decide which way to follow when
I have to write a simple function.

For example, I want to write a function which generates a random string. In
an OOP environtment, it is a matter of course to create a static class and
a static method for that. But why? Isn't it more elegant, if I implement
such a simple thing as a plain function? Not to mention that a function is
more efficient than a class method.

So, in object-oriented programming, what is the best practice to implement
such a simple function?