Hard-boiled eggs rise up when they are spun, whereas raw eggs remain flat. The mystery of why this is so has been cracked in time for Easter by Cambridge mathematician Professor Keith Moffatt, and Japanese physicist Professor Yutaka Shimomura.

In research published this week in the journal Nature, the pair explain how the phenomenon is caused by the weak friction between the egg and the table on which it spins.

Professor Shimomura explained: "If we spin a hard-boiled egg on its side, it mysteriously rises to spin on one end. This is because when a cooked - or solid chocolate - egg spins, it slips a little, producing a frictional force between the shell and surface.

"This force destabilises the spinning egg and kick-starts it to rise, raising the egg's centre of gravity. A part of the kinetic energy which spins the egg in the horizontal plane is converted to potential energy to raise it up."

A raw egg does not produce the same effect because the liquid inside absorbs and dissipates the energy, leaving any gyroscopic forces generated too weak to stand the egg on end.

Professor Moffatt is the former Director of the Isaac Newton Institute for Mathematical Sciences. Professor Shimomura is a visiting professor in the Hopkinson Laboratory, Department of Engineering. The two spent six months of their spare time working on the project.

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