human nature

The one Aesop’s fable I come back to again and again is the one about the frog and the scorpion (which I understand might not even be genuine Aesop, but whatever):

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The Scorpion and the Frog (from the Aesop’s Fables page):

A scorpion and a frog meet on the bank of a stream. The scorpion asks the frog to carry him across on its back. The frog asks, "How do I know you won't sting me?" The scorpion says, "Because if I do, I will die too."

The frog is satisfied, and they set out. But in midstream, the scorpion stings the frog. The frog feels the onset of paralysis and starts to sink, knowing they both will drown, but has just enough time to gasp "Why?"

Replies the scorpion: "It's my nature..."
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It just explains so much.