Zen Story (V)

Two monks were washing their bowls in the river when they noticed a scorpion that was drowning. One monk immediately scooped it up and set it upon the bank. In the process he was stung. He went back to washing his bowl and again the scorpion fell in. The monk saved the scorpion and was again stung. The other monk asked him, "Friend, why do you continue to save the scorpion when you know it's nature is to sting?"
"Because," the monk replied, "to save it is my nature."

Why do we do foolish things when we know that what we are doing is self destructive even before we do them ?