  We prove that Jardine's model category of simplicial presheaves can be
obtained by localizing the `discrete' version at the collection of all
hypercovers. One consequence is that the fibrant objects can be explicitly
identified in terms of a hypercover descent condition. Another is a very simple
approach to change-of-site functors. In an appendix, we discuss how this
hypercover localization compares to the more naive process of localizing at the
Cech complexes; the two are not the same in general, but agree in some cases of
interest.
