In the days of auld lang syne on Earth-that-was, a scorpion was creepy-crawling along a riverbank, wondering how to get to the other side. It came across an animal that could swim: some versions of the tale say it was a fox, others report a quokka. I'm going to assume it was a fox.
So the scorpion asks the fox to take it on her back and swim across the river. What does the fox say? She says, "No." The scorpion says, "If this is because you're afraid I'll sting you with my near-instantly-fatal toxins, don't worry—if I did that, then we'd likely both drown. By backwards induction, you're safe." What does the fox say? After pondering for a few moments, she says, "Okay."
So the scorpion gets on the fox's back, and the fox begins to swim across the river. When the pair is halfway across the river, the scorpion stings the fox.
The fox howls in pain while continuing to paddle. "Why?!" she cries. "Why did you do that?! As you said before, now we're likely to both drown."
The scorpion says, "I can't help it. It's my nature."
As the fox continues to paddle, the scorpion continues. "Interestingly, there's a very famous parable about this exact scenario. There was even an episode of Star Trek: Voyager titled after it. As a fox who knows many things, you must have heard it before. Why did you believe me?"
"I can't help it," gasped the fox, who might after all have been a quokka, as the poison filled her veins and her vision began to blur and her paddling began to slow. "It's my nature."