Particle separation is an important but often neglected topic in undergraduate curricula. This article discusses how the method of gravitational field-flow fractionation (GrFFF) can be used to illustrate many principles of separation science and some fundamental concepts of physical chemistry. GrFFF separates particles during their elution through a flat rectangular channel. The mechanism can be readily understood in terms of gravitational settling, laminar fluid flow, and hydrodynamic lift forces. The article outlines material suitable for lecture and laboratory courses in analytical and physical chemistry. A tested laboratory experiment is available. (Contains 10 figures.)