  The principal aim of this note is to give an elementary proof of the fact
that any two fiber functors of a Tannakian category are locally isomorphic.
This builds on an idea of Deligne concerning scalar extensions of Tannakian
categories and implements a proof strategy which Deligne attributes to
Grothendieck. Besides categorical generalities, the proof merely relies on
basic properties of exterior powers and the classification of finitely
generated modules over a principal ideal domain.
  Using related ideas (but less elementary means) we also present an
alternative characterization of Tannakian categories among the more general
weakly Tannakian categories. As an application of this result we can construct
from any right exact symmetric monoidal abelian category with simple unit
object a universal Tannakian category associated to it.
