system
Given the following post, respond as if you're commenting on Reddit:
human
Why do objects make a sound when they are broken? What is causing the emission of the sound?

Say I break a carrot and you hear that "snap". What is causing that sound? Why is it making that sound?
gpt
You hear sound when *rigid*\* objects are *suddenly* broken. Cut butter or rubber and you don't hear any sound, even if you cut quickly. Similarly, cut glass very slowly with a knife and you don't hear anything (cut, not scrape). When you suddenly break an object, right before breaking, the molecules close to the fracture lines are stretched away from their equilibrium points. In a rigid object, when the break happens these molecules very quickly snap back to their equilibrium points because they are pulled back by their regular chemical bonds to neighboring molecules. The more rigid and less dense the object, the more quickly the molecules snap back once the fracture happens. But when a molecule reaches its equilibrium point, it now has momentum and overshoots the equilibrium point, smashing into neighboring molecules. These neighboring molecules are then pushed into their neighboring molecules before snapping  back to their equilibrium points. A wave of molecules pushing and snapping back therefore traverses through the object. We call this systematic waving of molecules a "mechanical vibration" or "sound". When the vibrations traveling through the material reach the surface of the object, the vibrating molecules smash into the molecules of air and get the air itself vibrating. The vibration (i.e. the sound) travels through the air into your ear and you perceive the sound.

In order for the sound in the material to be audible to a human, it has to be high enough frequency and high enough amplitude. This will only happen for the sound created by breaking an object if the object is rigid enough and you break it fast enough. 

\* Note that even a non-rigid object such as a rubber band can make a sound if you stretch it enough so that it is pseudo-rigid. Cut a limp rubber band with scissors and you don't hear a sound. But stretch a rubber band as much as possible and then cut it, and you will hear a snap. All of the above concepts still apply because a non-rigid object that is stretched enough acts in some ways like a rigid object.