[Physics] Why glass always breaks into sharp pieces

Question

I understand that common glass and pottery (though not, for example, the glass of a car's windscreen) breaks very easily because it is very brittle and it has a great number of "cracks" in its structure.

But why sharp corners are always formed?

Answer 1

Ordinary glass has an irregular 'amorphous' molecular structure. That means that every pane has a unique, random 'fingerprint' of weak points, at which the glass might shatter.

When it shatters, it breaks along lines connecting the weak points. Sharp-edged polygons are the inevitable result of cracking along random lines on a pane of glass.

However, modern 'tempered' glass may be engineered for safety. It cracks into something resembling gravel, that isn't that sharp and won't cause deep cuts. Have you ever seen a vandalized telephone box or bus shelter? If so, you may have noticed the glassy gravel formed by the smashed glass.