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Why the biggest nuts always end up on top
💡 In a shaken pile of grains, large particles rise because small ones keep filling the gaps beneath them — a process called granular convection.
Shake a can of mixed nuts and something strange happens: the large Brazil nuts rise to the top while the tiny peanuts sink. This is so reliable that scientists gave it a name — the Brazil nut effect, a form of granular convection. It seems to defy common sense, because heavier objects 'should' sink, not float.
The secret is that a jostled pile of grains behaves less like a solid and more like a strange fluid. Each shake momentarily opens tiny gaps beneath the particles. Small grains easily trickle down into these gaps, so they gradually work their way to the bottom. Large grains can't fit through, so they can only ever move up — and once up, they can't fall back down. Over many shakes, the big pieces get ratcheted to the surface.
This isn't just a snack-time curiosity. Granular materials — sand, cereal, pills, gravel, powdered medicine — surround us in industry, and unwanted size separation (called segregation) ruins products that need to stay evenly mixed, from concrete to breakfast cereal. Engineers study the effect to keep mixtures uniform, while planetary scientists use it to explain why asteroids and rubble-pile bodies push their largest boulders to the surface after countless small impacts.