semantics

why are people who work on mAchines called mEchanics? why the sudden change of vowels. makes no sense to me.

and imagine a guy died, say, percy, and two other guys are talking about him.

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situation 1
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GUY 1: did percy die last year?
GUY 2: yeah.

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situation 2
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GUY 1: didn't percy die last year?
GUY 2: yeah.

you add a negative but the answer doesn't change. fancy that.