Leigh's discussion of springs reminded me of an amusing true story.
My father frequently does volunteer work at the local science exploratorium
in Atlanta. And one day he was talking with a group of students of age 8-11
roughly. Somehow he got around to showing them his wrist watch, which is of
the wind-up variety. The students were absolutely amazed and in fact
thought this was the latest manifestation of modern technology. One student
exclaimed, " imagine that, what will they think of next; you don't even have
to replace the batteries on that watch".