  Christol's theorem characterises algebraic power series over finite fields in
terms of finite automata. In a recent article, Bridy develops a new proof of
Christol's theorem by Speyer, to obtain a tight quantitative version, that is,
to bound the size of the corresponding automaton in terms of the height and
degree of the power series, as well as the genus of the curve associated with
the minimal polynomial of the power series. Speyer's proof, and Bridy's
development, both take place in the setting of algebraic geometry, in
particular by considering K\"ahler differentials of the function field of the
curve. In this note we show how an elementary approach, based on diagonals of
bivariate rational functions, provides essentially the same bounds.
