Muesli lovers have long pondered why the heavier (and tastier) components of their breakfast regularly appear at the top of the packet, a baffling question often referred to in scientific circles as the "Brazil nut effect".

Ten years ago, a team of scientists at the Institute for Food Research in Norwich, UK, said they had solved the mystery. Demonstrating their observations with computer simulations, they argued that the segregation was caused by a complex combination of statistics and gravity. When Brazil nuts rise they leave space that the smaller oat flakes in the packet can easily slid into, they said, thus the nuts rose while the flakes fell.

On the other hand, many more flakes would have to move together to make way for a nut to fall.

According to new research published today in the Nature journal, however, air also plays an important role in determining the pace at which Brazil nuts reach the top spots.

From the University of Chicago, Matthias Mobius, Prof Sidney Nagel and Prof Heinrich Jaeger report that the interaction of air with the smaller cereal bits was important, as air causes a drag on the particles. They suggest that if the conditions inside the cereal box offered the muesli a vacuum, all the cereal's components will rise at the same pace.