  In this paper, we classify the hypersurfaces of
$\mathbb{S}^2\times\mathbb{S}^2$ with constant sectional curvature. By applying
the so-called Tsinghua principle, which was first discovered by the first three
authors in 2013 at Tsinghua University, we prove that the constant sectional
curvature can only be $\frac{1}{2}$ and the product angle function $C$ defined
by Urbano is identically zero. We show that any such hypersurface is a parallel
hypersurface of a minimal hypersurface in $\mathbb{S}^2\times\mathbb{S}^2$ with
$C=0$, and we establish a one-to-one correspondence between the involving
minimal hypersurface and the famous ``sinh-Gordon equation'' $$
(\frac{\partial^2}{\partial u^2}+\frac{\partial^2}{\partial v^2})h
=-\tfrac{1}{\sqrt{2}}\sinh(\sqrt{2}h). $$ As a byproduct, we give a complete
classification of the hypersurfaces of $\mathbb{S}^2\times\mathbb{S}^2$ with
constant mean curvature and constant product angle function $C$.
