Why Hard-Boiled Eggs Spin (But Raw Ones Don't)

Here's a kitchen trick that's equal parts party trick and practical tool: spin an egg on your counter. If it rotates smoothly and quickly, you've got a hard-boiled egg. If it wobbles awkwardly and barely makes a full rotation, it's raw.

The reason has nothing to do with magic and everything to do with physics. Specifically, the difference lies in what's happening inside the shell.

Solid vs. Liquid: A Physics Showdown

A hard-boiled egg is essentially one solid object. The white and yolk have coagulated into a firm mass that moves as a single unit. When you spin it, all that mass rotates together in harmony, creating smooth, sustained motion.

A raw egg, on the other hand, is a liquid-filled vessel. The whites and yolk slosh around inside the shell independently. When you try to spin it, you're only rotating the shell—the gooey insides resist the motion, creating drag and causing that characteristic wobble.

Think of it like spinning a water bottle. A frozen water bottle spins beautifully. A half-full one? Not so much.

The Stop-and-Go Test

Want to make this test even more definitive? There's an advanced version. Spin the egg, then briefly stop it with your finger and immediately release. A hard-boiled egg stays stopped—all its mass halted at once. A raw egg starts spinning again, because the liquid inside was still in motion and transfers momentum back to the shell.

It's like stopping a merry-go-round with people on it. Stop the platform, and the people keep moving.

Why This Actually Matters

Beyond being a neat science demonstration, this trick has legitimate kitchen utility. Ever boiled a batch of eggs and forgot to mark which ones were done? No need to crack them open. A quick spin test on the counter reveals all.

Some cooks also use this method to check if eggs have been accidentally frozen and thawed, though the results are less reliable in that scenario.

The Physics Behind It

The scientific principle at work here is rotational inertia (also called moment of inertia). It describes how an object's mass distribution affects its resistance to rotational motion. In a hard-boiled egg:

• Mass is evenly distributed and solid
• All parts rotate together as one rigid body
• Minimal internal resistance to motion

In a raw egg:

• Mass is liquid and can shift independently
• Internal friction between shell and contents creates drag
• Energy is lost to viscous resistance from the liquid

The soft-boiled egg mentioned in the original fact falls somewhere in between. The partially set white provides some structure, but the liquid yolk still creates enough internal resistance to prevent smooth spinning.

So next time you're staring at a carton of mystery eggs wondering which ones survived the boiling process, just give them a spin. Physics has your back.