-> As far as the challenge - a mechanical device which can't fail - how
-> about a rock? Technically, rocks, levers, inclined planes and so
-> forth count as mechanical devices. I suppose given a few thousand
-> years one of those would fail too, though.
Ever try to hammer a nail with a rock when you were a kid, Steve?  Pick
up the WRONG kind of rock and it will break in your hand when you strike
the nail. 
Regards
John
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