Two eggs look exactly alike. You have only a table and your hands, yet one brief motion test is enough to reveal what each shell contains.
Raw or Hard-Boiled?
Riddle statement
You have two eggs whose exteriors look identical.
One is raw. The other is fully hard-boiled and has cooled down.
You may not crack them, open them, shake them near your ear, weigh them, heat them, wet them, or mark them.
You may only place them on a smooth table and touch them with your hand.
What test can you perform to distinguish them reliably?
Show solution
Solution
Answer: spin each egg, stop it briefly with one finger, and release it immediately.
1. The test
Perform the following procedure on each egg:
- spin it on a smooth table;
- touch the shell for an instant to stop it;
- remove your finger immediately.
The expected results are different:
- the hard-boiled egg remains still;
- the raw egg tends to resume spinning or make an additional small movement.
This second observation is the more reliable test.
2. What happens inside the hard-boiled egg
In a fully hard-boiled egg, the shell and solid interior behave approximately as one rigid body.
When your finger stops the shell, it also stops the rest of the egg. Once your finger is removed, no internal part is still rotating strongly enough to set the shell moving again.
The hard-boiled egg therefore remains still.
3. What happens inside the raw egg
In a raw egg, the shell rotates around a liquid interior.
When you touch the egg briefly, you mainly stop the shell. The liquid inside does not stop instantaneously: because of inertia, it retains part of its motion.
After you remove your finger, the moving liquid transfers rotational force back to the shell through internal friction. The egg may therefore begin rotating again.
In physical terms, the liquid retains some angular momentum while the shell is briefly held still.
4. An initial visual clue
You will also commonly observe that:
- the hard-boiled egg spins faster and more steadily;
- the raw egg is harder to spin and tends to wobble.
In the raw egg, the liquid does not begin rotating in perfect unison with the shell. This relative motion dissipates energy and makes the spin less stable.
However, this comparison can depend on the exact shapes of the eggs and on how they are spun. The decisive procedure is therefore spin, stop briefly, and release.
Key idea: stopping the shell does not immediately stop the liquid inside it. The hidden motion remaining inside the raw egg gives it away.