By combining the Extreme Value Theorem and Fermat's Theorem, we
get
Rolle's Theorem: Let $f$ be
a function that is continuous on a closed
interval $[a,b]$ and differentiable on the
open interval $(a,b)$, and suppose that
$f(a)=f(b)=0$. Then there is a point $x=c$,
somewhere between $x=a$ and $x=b$, such that
$f'(c)=0$.
Roughly speaking, the Extreme Value Theorem says that there has to be
a maximum and a minimum. If it's in the interior, Fermat's Theorem says
that it has to be a critical point. Since $f$ is differentiable on the
interior, it has to be a point where $f'(c)=0$. The details of this
argument, as well as the cases where the maxima and minima are at the
endpoints, are found in the following video: