Well, unless you have very special science-defying cereal, they don’t.
Despite logic leading us to believe that big, presumably heavy things sink due to the force of gravity, brazil nuts mysteriously make their way to the top of cereal packets. This phenomenon is such a strange one that scientists have coined a phrase to try to explain it: granular convection.
The brazil nuts and other large and small bits in your cereal are the ‘granules’ and it is thought that they move up in a way similar to convection, hence the name. Convection is that thing where warm air rises up from toasty radiators to the top of a room, cools and then sinks back down to be heated again. You can see the same thing happening when you heat up soup in a pan. Essentially, it’s all about movement up and down, and your cereal goes through plenty of that.
From the moment that your cereal is thrust into its packet, it has a fairly traumatic life. It’s buffeted around until it reaches a packing crate, is shoved on the back of a lorry (or even a boat or a plane), where it is further jolted about until it reaches the supermarket. Once there, a shelf-stacker half-heartedly plonks it on a shelf and minutes later you roll up, plucking it back up again and dropping it haphazardly into your trolley. After a jolting trip around the supermarket, you shove it into a bag, throw it into the boot of your car and jostle it home. And then you have the audacity to shake the poor thing before opening it and probably every day after that.
All of that vibration and movement causes the cereal inside the box to move around. The cereal ‘flows’ up through the middle of the box because this is where the upward vibration is strongest. As bits of cereal move up in the middle, the spaces they left behind must be filled, meaning that bits at the top then trickle back down the sides to fill the gaps. As this happens, the brazil nut – the giants of the muesli world – pile up on the top whilst the smaller, craftier raisins slip through small gaps at the sides and vibrate themselves back down to the bottom. Brazil nuts are simply too big to fit in these tiny gaps and find themselves relegated to the top. Or at least that’s one explanation.
Some scientists have a different theory and believe it is all to do with order, not convection. Air pockets collect around those pesky bulky brazil nuts, resulting in wasted space. When the packet is shaken, physics steps in to fill in all those air pockets by bringing down the smaller bits of cereal. As a result, the brazil nuts end up on top and the universe is happy because the laws of physics are being obeyed again.
Whichever explanation is correct, the end result is always the same: big brazil nuts end up on top thanks to small bits of cereal sneaking into the gaps that the nuts can’t fit into – all thanks to the box being shaken about so much. Which also means that shaking your box of cereal every morning is pointless. Unless you’re vigorously shaking it upside down. In which case, you might be able to reverse the effect long enough to get perfectly distributed cereal. But hasn’t the muesli suffered enough already?