More often than not, we're going to want to give our modules their own files. It's just nicer to keep things separated and contained as much as possible, because it helps keep the code manageable. Fortunately, this is just as easy. In our .rs file, we can just write something like the following:

pub mod module_b;

That looks a lot like the previous example, except that it doesn't have the module contents right there between { and }. Instead, the Rust compiler goes looking for a file called either module_b.rs or module_b/mod.rs, and uses the whole file as the contents of the module_b module. So, if the file contains a couple of functions similar to the ones we saw previously:

pub fn a_thing() {
    println!("This is a module_b thing");
}

pub fn a_second_thing() {
    a_thing();
    println!("This is another module_b thing");
}

Then module_b will contain two functions named a_thing and a_second_thing. It's not a problem that those functions have the same names as functions in the module_a module from before, because they're in a different module.