Recent experiments show that the impact of a jet of non-cohesive
granular particles produces a highly collimated ejecta sheet.  The
angle of ejection agrees quantitatively with that produced by the
impact of a water jet.  An analogous behavior, the emergence of highly
collimated jet upon oblique impact of two gold ions, has been cited as
evidence for the liquid nature of the quark-gluon plasma at RHIC.
Both examples share the common feature that a system of particles
exhibits a liquid-like collective response even though the particles
are not confined together nor experience attraction towards each
other.  We analyze the impact of two granular jets.  The velocity
field and stress distribution within the granular jet agree
quantitatively with that of a perfect-fluid flow.  Our results show
that this close correspondence does not require that the system of
particles be in a liquid phase possessing a Newtonian viscosity. The
only requirements are that the collective motion is approximately
incompressible and that the shear stresses within the jet are small
relative to the normal stresses.