We're taught from an early age that heavy things, which are usually big things, sink to the bottom. Yet whenever you open a can of mixed nuts, the Brazil nuts always seem to be sitting on top, as if to say, "hello there—you didn't think the biggest nuts in the can would be sitting on top of smaller nuts waiting to greet you, challenging your concept of physics and the natural world, wouldn't you?" (Or is that voice just in my head?)
There is some interesting science going on here, in particular a phenomenon called granular convection, AKA The Brazil Nut effect. Granular convection is when small, solid, different-shaped bits of similar mass exhibit circulation patterns similar to fluid convection.
Still confused? Here's a more plain talking explanation:
As the can of mixed nuts goes from the factory to a warehouse then finally to the retail shelf, it is subjected to lots of vibration and friction, which causes the nuts to move around.
Each time the nuts are jostled in an upward motion, gravity ensures they come back down and the center of mass of the system constantly changes. The largest nuts, the Brazil nuts, are constantly pushed up as the smaller nuts continue to shift place. Their smaller size makes it easier for them to fall into the spaces under the larger particles. Over time, the larger particle rises in the mixture.
Have you ever notices this effect in other foods? Cereal perhaps? It's sometimes called the "Muesli Effect" for the same reason, as over time, some cereal mixtures seem to get sorted into layers of similar ingredients as granular convection takes place as the products go through the distribution process.