  In the paper by Klainerman, Rodnianski and Tao
\cite{Klainerman-Rodnianski-Tao}, they give a physical space proof to a
classical result of Klainerman and Machedon \cite{Klainerman-Machedon} for the
bilinear space-time estimates of null forms. In this paper, we shall give an
alternative and very simple physical space proof of the same bilinear estimates
by applying div-curl type lemma of Zhou \cite{Zhou} and Wang-Zhou
\cite{Wang-Zhou-1}, \cite{Wang-Zhou-2}. As far as we known, the later
development of wave maps \cite{Sterbenz-1}, \cite{Sterbenz-2}, \cite{Tao-1},
\cite{Tao-2}, \cite{Tataru-1}, \cite{Tataru-2}, and the proof of bounded
curvature conjecture \cite{Klainerman-Rodnianski-Szeftel-1},
\cite{Klainerman-Rodnianski-Szeftel-2} rely on basic idea of Klainerman and
Machedon \cite{Klainerman-Machedon} as well as Klainerman, Rodnianski and Tao
\cite{Klainerman-Rodnianski-Tao}.
