Is the Egg Cooked? Give It a Spin and See

Q: Why does a hard-boiled egg spun on a tabletop rise on one end and spin like a top?

A: Researchers from Japan and England reported in the March 28 Nature that they spent six months filching eggs from their family refrigerators and trying to explain the mysterious forces controlling this behavior. Although it required a lot of math to solve the problem, the final cause was simple: friction.

Imagine an egg spinning on its side on a tabletop. Because of the curve of its shell, it is touching the table at just one point. But the contact point is not fixed; it slides in a small circle around an imaginary vertical axis. As the egg slides across the table, the friction created slows the egg’s rotation slightly, and the contact point with the table moves off-center. The egg begins to twist as it spins. One end slowly rises until the egg stands vertically. For a few seconds, anyway.

The egg can be any size or type. But it must be hard-boiled. When you try to spin a soft egg on a table the fluid inside lags behind the shell. By the time the fluid is spinning at the same rate as the shell, the egg doesn’t have enough kinetic energy to stand up.