Found this one on the internet a few years ago.
You've all heard the expression "girls require time and money."
Therefore, Girls = Time x Money
And "time is money" is common knowledge. Thus,
Time = Money
Girls = Money^2
It is said that "money is the root to all evil." Thus,
Money = SquareRoot[Evil]
Money^2 = SquareRoot[Evil]^2
Money^2 = Evil
And so, we must conclude that
Girls = Evil
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A boy walked into a bar, and met up with the owner of a famous local stable. His best horse, Hannibal, was said to react to no man, despite being a circus horse. Yet, the boy claims he can make the horse weep. And so, the boy, the stable owner and a few farmers accompany the two to the stables.
And there they find Hannibal.
"Go ahead," says the stable owner. "Work your magic."
So the boy walks up to Hannibal, and whispers something in his ear. All of a sudden, the horse cries! Flabbergasted, the stable owner stares, and asks the boy if he can make the horse laugh during their circus show the following day. The boy agrees.
The next day, the boy walks up to Hannibal again, and whispers yet again in his ear. In front of everyone, the horse starts to laugh happily! After the show, the stable owner simply must ask the boy why he could make it laugh.
"First time, sir," says the boy, "I told the horse that mister's penis here was bigger than the horse's."
The stable owner stares at the boy in disbelief.
"Second time around, sir," continues the boy, "I told him I was lying."