Now, the following proof I seen at work today. I must stress at this point now that I DO NOT AGREE IN ANY WAY IN WHAT IT PROVES, however I do think it's pretty clever, so I decided to upload it to here.

It begins like this. Girls require time and money, according to this theorem. Therefore:

GIRLS = TIME * MONEY

It is also known (to anyone who is in any way connected to any business whatsoever) that

MONEY = TIME

Therefore:

GIRLS = MONEY * MONEY = MONEY^2 [MONEY squared]

Now, it is a popular fact of life that money is the root of all evil:

Money = SQRT(EVIL) [the square root of EVIL]

And, putting this into our formula, this reads:

GIRLS = (SQRT(EVIL))^2 [the square root of EVIL squared]

Which posthastely simplifies down to:

GIRLS = EVIL

QED

This therefore proves that girls are evil, according to this theorem. May I now repeat, however, that I do not necessarily agree with this and I do not believe that girls are any more eviler than men.