This paper is intended to study the large-time behavior of the second moment (energy)
of solutions to the porous medium equation. As we shall briefly discuss in the following,
the knowledge of the time evolution of the energy in a nonlinear diffusion equation is of
paramount importance to reckon the intermediate asymptotics of the solution itself when
the similarity is missing. Thus, the present study can be considered as a first step in the validation of a more general conjecture on the large-time asymptotics of a general diffusion equation.