The blades of the the grinder strike molecules, causing them to oscillate
and generating phonons which *flow* and interact with the other
molecules of brass. These inelastic collisions eventually approach
a Boltzmann equilibrium at an increased mean energy per degree of
freedom.
Now why didn't Rumford think of that?
What would your students make of it?
Perhaps historical views and terms more closely match the needs of our
students, at least at the introductory level.