Rule of the what now?

It often surprises me when I discover that someone or a group of people don't know of a certain phrase, phenomenon and just general knowledge that I believed to be public domain which everybody was aware of. I don't consider myself a close student of current affairs but I seem to always be up on the latest goings on and developments in languages and technologies (chavs, MySpace, etc) that are occurring. Others just don't seem to be as up to date.

Today, for example, I nipped through to see the receptionist about something. When I got there she was on a break so reception was being covered with a young lass of a similar age to me. We had a quick chat and as she was leaving she told me of a piece of handwritten work which was being typed up, but she was confused by it. I offered to help and she showed me a document which detailed ladder usage in the workplace (yawnsome I know, but everything stops for Health and Safety). The sentence she was struggling with went something like this:

"The general rule of thumb is to never stand on the top step of ladders or a stepladder"

Having read it, the following conversation ensued:

Me: What's the prob?
Her: Does that say 'thumb'?
Me: Yeah
Her: Why?
Me:?
Her: Why does it say 'thumb'? That doesn't make any sense.
Me: It says 'rule of thumb'. Have you never heard of it?
Her: Nope
Me: Eh?

This continued for a couple more hours as I couldn't believe she hadn't heard of the phrase 'rule of thumb'. Shortly after the receptionist returned, a middle-aged lady, and she confirmed that she'd never heard of it either. I was absolutely amazed that I had to explain what I considered to be a standard piece of spoken English to people I considered to be on the ball.

It got worse though. I returned to my office and recounted my experience to my colleagues and bugger me if they hadn't heard of it either. I couldn't believe it, to the extent that I thought only I and the writer had ever come across it. Luckily the boss had heard of it so my pending breakdown was avoided. I now plan to poll the remainder of the building to get an idea of just how out of touch they are. Oddballs.

Ironically, as a reszult of this experience, there is a rule of thumb to follow in future: Don't assume everybody has heard of it just because you have.