  We use porosity to study differentiability of Lipschitz maps on Carnot
groups. Our first result states that directional derivatives of a Lipschitz
function act linearly outside a $\sigma$-porous set. The second result states
that irregular points of a Lipschitz function form a $\sigma$-porous set. We
use these observations to give a new proof of Pansu's theorem for Lipschitz
maps from a general Carnot group to a Euclidean space.
