The scorpion and the frog

A scorpion that wanted to get across a pond asked a frog for help. The frog refused knowing that the scorpion would sting him. But the scorpion insisted until the frog agreed. Halfway across the pond, the scorpion could resist the urge no longer and he stung the frog. The frog asked why and the scorpion said he couldn't resist, it was in his nature. And then they both drowned.