You’ve probably seen this happen.

A car windshield is frozen.
Someone pours hot water on it.
The glass cracks instantly.

It looks random, but it isn’t.

Glass expands when it heats up.
When hot water hits frozen glass, the surface heats and expands immediately, while the inner layer stays cold.

That creates uneven expansion.

The glass can’t stretch fast enough to handle that stress, so it cracks to release the pressure.

The key thing most people miss is speed.
If the temperature change happens slowly, glass can adjust. When it’s sudden, it fails.

That’s why defrosters work — and boiling water doesn’t.

— How Do Things Work