Why glass always breaks into sharp pieces? 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? 
 A: 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.
A: Say you have a piece of glass which is a thick surface (this is the case for drinking glasses, lightbulbs, window glasses...) that you break. Necessarily with every smooth piece there is a sharp piece that comes with it. Or you could say that the pieces must fit one another in the end, and the convex (smooth) part produces a concave (sharp) part :

There may be an explanation on the microscopic scale, and this is probably not enough. If someone has an answer I would also be interested in that !
