Why is there a sharp cracking sound when we break a plastic ruler? The question appears to be simple and common but I need a apropriate answer.
When you bend a plastic ruler you are stressing the molecular/atomic structure of the material (polymer). If you put something into mechanical stress you are putting such system into a higher energy state that when it is not.
If you stress it enough it will break and liberate most of that energy "at once" in the form of mechanical vibrations (a.k.a. sound).