  In Fritz John's famous paper (1979), he discovered that for the wave equation
$\Box u=|u|^p$, where $1<p<1+\sqrt{2}$ and $\Box$ denoting the d'Alembertian,
there is no global solution for any nontrivial and compactly supported initial
data. This paper is intended to simplify his proof by applying a Gronwall's
type inequality.
