Question

When we remove unwanted body hair by plucking/shaving, we knowthat they grow back more or less to its natural length.

When we trim them, the hair also seem to grow back to itsnatural length, but this has mystified me for some time.

a) How does the hair know it's been cut without stress caused tothe hair follicle, and therefore start growing again; and

b) How does it know when to stop growing when it reaches theintended length?