Whenever someone asks for an explanation of something, what they expect in return is a nice story. A story that tells you about something that happened in the past, and what happened after that, and what that means now. So if you asked: Why do rainbows appear after a storm? Then I could say: It's because God put it in the sky to tell us that he's not going to send us another flood. It's a great story: It tells you what happened, why it happened, and what it means now. But it's just a story - you know it's just a story because you know the scientific explanation for rainbows, and that it's just a boring case of refraction of sunlight through water droplets.
So when I say: A good rule of thumb is to stay two seconds behind the car in front, what I mean is: This is a useful and approximate way of keeping a safe distance from the car front, because it would be impractical to continually measure the exact distance. That's what 'rule of thumb' means. But you might say: Rule of thumb - I wonder why we say that? And then you'd be expecting me to tell you a story. You don't want to hear that the origin is lost in the mists of time. So I clear my throat and say:
In days of yore, it was the rule that when a servant retired, he was allocated a piece of land by his master. And the master would hold up his thumb at arms length, and the parcel of land that was covered by his thumb would become the servant's. And that's why today we have the saying: Rule of thumb.
And you'd say: Is that true? And I'd say:
No - it's just a story - I made it up. But why let the truth get in the way of a good story.