How To HttpContext.Current Work, Providing Context Appropriate State Off A Static Variable

I've always wondered how you can access the correct state of the current http context via a static method:

HttpContext.Current.Session["foo"] = "bar";

In any other program, anywhere else, my initial assumption about working with a static accessor like this is that changing it will change it across all threads. Similarly, another thread running my change it on me while I am trying to use it.But HttpContext.Current does not behave like this. It provides the appropriate state for the given request, even through the static accessor.