another way to think of crystals is as a mechanical resonator (like a guitar
string). you can get a guitar string to resonate at an 'overtone' by carefully
picking the length, and applying a mechanical impulse (plucking the string)
then removing your finger
in fact, some low frequency 'crystals' (down around 32khz) are actually
ceramic 'resonators' since a quartz crystal for very low frequencies are
very big.
