Science Puzzle
Why Does a Car Window Shatter into Little Cubes?
When a car's side window is smashed, it doesn't break into long, sharp shards like a broken bottle. The whole pane suddenly crumbles into hundreds of little blunt cubes, and they rain down onto the seat and the road.
Why does a car's side window shatter into little cubes?
The Answer
It's toughened glass, made on purpose so it's hard to break and, when it does break, falls into small blunt pieces that are far less likely to cut the people inside.
Toughened glass is heated until it's nearly soft, then its surfaces are blasted with cold air. The outside sets hard first. As the inside cools later, it shrinks and pulls on the hardened surfaces, leaving them squeezed tight while the middle is stretched. Glass usually breaks when a tiny scratch on the surface is pulled open; a squeezed surface pushes scratches shut, so toughened glass is several times stronger than ordinary glass.
All that squeezing and stretching is stored energy. Once a crack does get through the squeezed surface into the stretched middle, the whole pane lets go at once and shatters into small cubes with blunt edges.
Windscreens work differently. They're laminated: two layers of glass glued to a tough, clear plastic sheet in the middle. A stone can crack a windscreen, but the plastic holds the pieces together, so it stays in place and keeps people in and flying objects out. Toughened glass is also used for shower screens and oven doors, anywhere a sharp shard would be dangerous.
The principle: Toughened glass. Cooling glass quickly locks its surface in compression, which makes it much stronger and, when it does break, releases the stored stress so it shatters into small, blunt pieces.