Classical dynamics: spinning eggs--a paradox resolved.

If a hard-boiled egg is spun sufficiently rapidly on a table with its axis of symmetry horizontal, this axis will rise from the horizontal to the vertical. (A raw egg, by contrast, when similarly spun, will not rise.) Conversely, if an oblate spheroid is spun sufficiently rapidly with its axis of symmetry vertical, it will rise and spin about the vertical on its rounded edge with its axis of symmetry now rotating in a horizontal plane. In both cases, the centre of gravity rises; here we provide an explanation for this paradoxical behaviour, through derivation of a first-order differential equation for the inclination of the axis of symmetry.