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There once lived a scorpion and a frog.

The scorpion wanted to cross the pond, but, being a scorpion, he couldn’t swim. So he scuttled up to the frog and asked:

“Please, Mr. Frog, can you carry me across the pond on your back?”

“I would,” replied the frog, “But, under the circumstances, I must refuse. You might sting me as I swim across.”

“But, why would I do that?” asked the scorpion, “It is not in my interests to sting you, because you will die and then I will drown.”

Although the frog knew how lethal scorpions were, the logic proved quite persuasive. Perhaps, felt the frog, in this one instance the scorpion would keep his tail in check. And so the frog agreed. The scorpion climbed onto his back, and together they set off across the pond. Just as they reached the middle of the pond, the scorpion twitched his tail and stung the frog. Mortally wounded, the frog cried out:

“Why did you sting me? It is not in your interests to sting me, because now I will die and you will drown.”

“I know,” replied the scorpion, as he sank into the pond. “But I am a scorpion. I have to sting you. It’s in my nature.”

http://www.bluejake.com/archives/2004/07/07/some_frogs.php