  We study the existence of sign-changing solutions to the nonlinear heat
equation $\partial _t u = \Delta u + |u|^\alpha u$ on ${\mathbb R}^N $, $N\ge
3$, with $\frac {2} {N-2} < \alpha <\alpha _0$, where $\alpha _0=\frac {4}
{N-4+2\sqrt{ N-1 } }\in (\frac {2} {N-2}, \frac {4} {N-2})$, which are singular
at $x=0$ on an interval of time. In particular, for certain $\mu >0$ that can
be arbitrarily large, we prove that for any $u_0 \in \mathrm{L} ^\infty
_{\mathrm{loc}} ({\mathbb R}^N \setminus \{ 0 \}) $ which is bounded at
infinity and equals $\mu |x|^{- \frac {2} {\alpha }}$ in a neighborhood of $0$,
there exists a local (in time) solution $u$ of the nonlinear heat equation with
initial value $u_0$, which is sign-changing, bounded at infinity and has the
singularity $\beta |x|^{- \frac {2} {\alpha }}$ at the origin in the sense that
for $t>0$, $ |x|^{\frac {2} {\alpha }} u(t,x) \to \beta $ as $ |x| \to 0$,
where $\beta = \frac {2} {\alpha } ( N -2 - \frac {2} {\alpha } ) $. These
solutions in general are neither stationary nor self-similar.
