Why are the nuts in a box of muesli often at the top?

People have been pondering this apparent violation of the law of gravity since at least the 1930s and for many years scientists thought the answer was simply one of geometry. That is, the nuts may be heavier but they’re also larger than the other ingredients, which can thus move into the spaces created below the nuts as the box is shaken.

More detailed analysis has shown, however, that friction with the sides of the box is also important, producing a downward flow as the box is shaken. As smaller ingredients find it easier to work their way towards the sides, they tend to feel this effect most strongly, again leaving the larger stuff like nuts higher up. It's science, don't you know!