  The Euler equations of ideal gas dynamics posess a remarkable nonlinear
involutional symmetry which allows one to factor out an arbitrary uniform
expansion or contraction of the system. The nature of this symmetry (called by
cosmologists the transformation to supercomoving variables) is discussed and
its origin clarified. It is pointed out that this symmetry allows one to map an
explosion problem to a dual implosion problem and vice versa. The application
to laboratory simulations of supernova remnants is considered; in principle
this duality allows the complete three-dimensional evolution of highly
structured explosion ejecta to be modelled using a static target in an
implosion facility.
