  We show by means of an exact numerical approach that the momentum
distribution of a free expanding gas of hard-core bosons on a one-dimensional
lattice approaches to the one of noninteracting fermions, acquiring a Fermi
edge. Yet there is a power-law decay of the one-particle density matrix
$\rho_x\sim 1/\sqrt{x}$, as usual for hard-core bosons in the ground state,
which accounts for a large occupation of the lowest natural orbitals for all
expansion times. The fermionization of the momentum distribution function,
which is not observed in equilibrium, is analyzed in detail.
