Just For Fun...

Q:  

Why does a piece of chalk produce a hideous squeal if you hold it incorrectly? Why do squeaky doors squeak? Why do tires squeal on a car that is drag racing from a dead stop?

A:  

The squeal heard in these several situations results from ‘stick and slip.’ For example, the chalk first sticks on the chalkboard, but when the writer bends the chalk sufficiently, it suddenly slips and then vibrates, periodically striking the chalkboard and producing the squeal we hear. As the vibrations decrease, the friction between the chalk and the board increases until the chalk sticks once again.