Most things get a little bigger when they are heated, even though you probably cannot notice it. If you were to place a glass in a pan of water and heat the water slowly, all the surfaces of the glass would expand evenly, making the whole glass a little bigger.
But if you pour boiling water into a cold glass, the inner surface of the glass gets the heat first and expands more quickly than the outside. The pressure from this heated, expanded inside surface is so strong and so sudden as it hits the outside of the glass that the whole glass breaks.
See also:
- Why Is Boiling Water In the Microwave Dangerous and How Does Superheated Water In the Microwave Explode?
- Why Does the Water Level Rise When a Lit Candle Standing In Water Is Covered by a Upturned Glass Jar?
- What Is Gnocchi, Where Did Gnocchi Come From, and Why Do Italian Potato Dumplings Sink In Boiling Water?
- Why Do the Authorities Recommend Drinking Eight Glasses of Water a Day and How Big Is a Glass?
- How Does Water In the Water Heater Always Stay Hot Even When You Use It?
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