How To Tell If An Egg Is Hard-Boiled Without Opening It

A hard-boiled egg makes for a nutritious snack, but disaster can strike when our timing is off. Boil too long and the yolk looks like a 90s mouse ball, but too little and cracking that baby open is going to make a mess of your worktop and delay snackisfaction. But what if I told you a simple trick involving your finger can tell you if an egg is hard-boiled or not?

Denaturation transforms a runny raw egg into the bouncy oval that is a hardboiled egg, as heat unfolds proteins in the albumen that then bond with each other. In a chicken egg, the albumen boils white, but curiously, a hard-boiled penguin egg stays clear

All of this chemistry is unfolding behind the opaque calcium carbonate shell, meaning the elusive albumen is primed to trick us into cracking eggs before their contents have had time to set. We might not be able to see through the egg, but your finger can tell you if it’s hard-boiled or not.

How to tell if an egg is hard-boiled without opening it

According to our good friend Google, it takes up to 10 minutes to hard-boil an egg, but if you want to tell if an egg is hard-boiled without opening it, here’s what you need to do:

Step one – spin your egg on a hard surfaceStep two – tap your spinning egg with one finger, briefly stopping it from spinningStep three – watch what your egg does next

Results: if the egg comes to a stop and stays stopped, it’s hard-boiled. If it comes to a stop and then continues to spin when you lift your finger, its contents are still runny – put that thing back in the boiling pot from whence it came unless you want to end up with egg on your face, literally and figuratively.


“This is all due to momentum,” explained STEM Newcastle, an outreach team from Newcastle University, in a series of Easter Eggsperiments. “When you spin the eggs, you spin their insides too. In the hard-boiled egg, the insides are fixed to the shell so it behaves as you would expect. In the raw egg, the insides continue to spin after you’ve stopped the shell. When you let go, the momentum of the spinning yolk carries the shell and the whole egg starts spinning again.”

So go forth and boil with confidence, safe in the knowledge that science can guide you towards an egg with integrity.

Leave a Comment