Why should -1 multiplied by -1 be plus 1 and not -1

From: >parr\(*> (gniKyruaL_at_tenretnitb.moc)
Date: 07/30/04 

Date: Fri, 30 Jul 2004 09:52:12 +0000 (UTC)

This is a genuine question, but not mine. Some years ago, I knew a
girl, of a certain age, who could not understand why. Nor could I
find a logical/mathematical argument to show her. It's not that she
rejected the agreed arithmetical result, it's just that her intuition
told her that two minuses should not become positive.

I didn't spend a lot of time in the process because we had mutually
interesting things to discuss.

This girl was no slouch, but did not have a mathematical aptitude.
In fact to reassure herself that she had something above neck level,
she had applied to and been accepted into Mensa.

So here's the problem or you logic buffs: How would you present a
case that
-1 x -1 = +1
to someone whose intuition told her that it really ought to equal -1?

(If this intuition versus logic problem bears an uncanny resemblance
to another topic, it may be intentional:) 

--
)>==ss$$%PARR(º>   Parr

Relevant Pages