Chalk

It happens every time the prof calls a group of students to the board to solve a physics problem. Someone holds the chalk wrong and sends chills up and down the spines of everyone in the class with that familiar classroom torture technique: "squeaky chalk."

Why does a piece of chalk produce that hideous squeal? According to the book, The Flying Circus of Physics (With Answers), squealing chalk results from the phenomenon of "stick and slip." Incorrectly held chalk actually sticks to the blackboard. But when the writer bends the chalk enough, it suddenly slips and vibrates, sporadically striking the chalkboard and producing that squeal we hear. As the vibrations decrease, the friction between the chalk and the board increases until the chalk sticks again and the torture begins once more.