How does Greg Young's Fluent Interface work?

Question

I'm trying to follow along this article: http://codebetter.com/gregyoung/2007/12/05/a-use-for-extension-methods/

I follow along fine up to this point:

Now that I have these I can decorate the Builder with extension methods like:

public static CostBuilder Cost(this Builder s)
{
    return new CostBuilder();
}

Where do I stick this piece of code?

EDIT: Per vlad's answer

I stuck this code where the Builder and Create class was

public static partial class BuilderExtensions
{
}

Then for any namespace that needs to have the builder added, I stuck this beneath the builder class in the same class file:

public static partial class BuilderExtensions
{
    public static CostBuilder Cost(this Builder s)
    {
        return new CostBuilder();
    }
}

Answer 1

As per MSDN, Extension methods can be defined "inside a non-nested, non-generic static class".

In this particular case, you can add it to a static BuilderUtilities class.

Answer 2

Extension methods must be placed in static classes.

public static class BuilderExtensions {
    public static CostBuilder Cost(this Builder s) {
        return new CostBuilder();
    }
}

You can change the public keyword in the BuilderExtensions declaration to internal to keep the extension methods invisible to consumers of your API.