A scorpion asking a frog to carry him across a river.
The frog is afraid of being stung during the trip, but the scorpion argues that if it stung the frog, the frog 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 explains that this is simply its nature.