The function
$$f(x)=\left\{\begin{array}{ll}{x,} & {0 \leq x<1} \\ {0,} & {x=1}\end{array}\right.$$
is zero at $x=0$ and $x=1$ and differentiable on $(0,1),$ but its derivative on $(0,1)$ is never zero. How can this be? Doesn't Rolle's Theorem say the derivative has to be zero somewhere in $(0,1) ?$ Give reasons for your answer.