  Low-momentum excitations of a dilute Bose-Einstein condensate behave as
phonons and move at a finite velocity v_s. Yet the atoms making up the phonon
excitation each move very slowly; v_a = p/m --> 0. A simple "cartoon picture"
is suggested to understand this phenomenon intuitively. It implies a relation
v_s/v_a = N_ex, where N_ex is the number of excited atoms making up the phonon.
This relation does indeed follow from the standard Bogoliubov theory.
