We find an empirical relationship between the initial speed of Coronal Mass Ejection (CME) and the potential magnetic field energy of the associated active region (AR) that closely resembles the Sedov relation between the speed of a blast wave and the blast energy. We conclude that it is the magnetic energy of an AR that drives the CME. The restructuring of the AR field lines in the corona which can push material with Alfven speed and thus inject energy into the plasma on a time scale shorter than the dynamical time of the corona, is a likely process that can drive the CME. The empirical relationship allows the prediction of the maximum speed of a CME that can result from an AR of a given magnetic energy.