  In this paper we study some properties of the torsion function with Robin
boundary conditions. Here we write the shape derivative of the $L^{\infty}$ and
$L^p$ norms, for $p\ge 1$, of the torsion function, seen as a functional on a
bounded simply connected open set $\Omega \subset \mathbb{R}^n$, and prove that
the balls are critical shapes for these functionals, when the volume of
$\Omega$ is preserved.
