The scorpion and the frog

A scorpion asks a frog to carry him over a river. The frog is afraid of being stung during the trip, but the scorpion argues that if it stung the frog, both would sink and the scorpion would drown. The frog agrees and begins carrying the scorpion, but midway across the river the scorpion does indeed sting the frog, dooming them both. When asked why, the scorpion points out that this is its nature. The fable is used to illustrate the position that no change can be made in the behaviour of the fundamentally vicious.