Low-momentum excitations of a dilute Bose-Einstein condensate behave as phonons and move at a finite velocity
vs. Yet the atoms making up the phonon excitation each move very slowly; ${v}_{a}=p/\stackrel{\ensuremath{\rightarrow}}{m}0.$ A simple ``cartoon picture'' is suggested to understand this phenomenon intuitively. It implies a relation ${v}_{s}{/v}_{a}\ensuremath{\approx}{N}_{\mathrm{ex}},$ where
Nex is the number of excited atoms making up the phonon. This relation does indeed follow from the standard Bogoliubov theory.