10 Mind-Blowing Facts About the Science of Boiling an Egg Perfectly

3. Cold Starts vs. Hot Starts: It's About Thermal Shock

Plunging cold eggs into furiously boiling water is a recipe for cracked shells. This happens due to thermal shock – the rapid expansion of the egg's contents and trapped air bubble stresses the shell beyond its limit. Starting eggs in cold water allows them to heat up gradually along with the water, reducing this stress. Bonus: many find cold-start eggs cook slightly more evenly, as the gentle temperature ramp allows heat to penetrate more uniformly before the outer white sets too quickly.