Gaudin＇s "Bubble Particle Theory" of froth flotation is unsatisfactory for modeling packed column flotation. Proposed is a new hypothesis entitled the "Up-down Escalator Model". Said model, applied to open and packed column flotation cells, explains why packed columns produce higher grades with higher yields. It also explains why the volumetric capacity of packed columns is four to eight times larger than that of open columns. It predicts that control strategies of packed columns will be easier to implement than in open or conventional packed column cells. Finally, plugging of packing is not a realistic threat.