It is a well-known phenomenon that writing on a blackboard with chalk sometimes produces a squeaking noise. The explanation is also well-known: the chalk alternates between sticking to the board and sliding over it. This is called the stick-slip phenomenon and underlies many other noises like those of grasshoppers and bowed instruments.
Breaking the chalk into shorter pieces fixes the problem. My assumption had been that the shorter piece still produces noise, but that the frequency is too high for a human to hear it. However, I recently realized that I wasn't sure about this, and a web search quickly brings up answers like the following:
In that answer, we read
Simple: break the chalk in half and use the 'broken' ends. Squeaks disappear!
This suggests that the rugged surface of the resulting pieces plays a crucial role. I don't recall that it ever made a difference which end of the piece I used, so I have my doubts about this explanation.
Question: why does the broken piece of chalk not produce an audible sound?
It would be especially nice to know if someone had tried writing with an unbroken chalk cylinder which was short to begin with, because it was produced that way.
Edit: slip-stick -> stick-slip