When a container of granular material is shaken, we might expect the largest particles to make their way to the bottom. Instead the opposite often happens: Vibrating a container of mixed nuts, muesli, or raisin bran often brings the largest (and presumably heaviest) items to the top.

Precisely why this happens is unclear. An irregularly shaped Brazil nut might “shoulder” its way above smaller nuts as it turns among them; the rising of large particles might help to lower the center of mass of the aggregate; or perhaps the size of the largest particles prevents them from descending in a container’s natural convection flow once they reach the surface. For now it’s an unsolved problem in physics.