I have spent the whole of my teenage and adult life believing that 30cm equals 12 inches - exactly, not approximately.
Today, measuring my latest piece of knitting, I realized I was wrong. I couldn't believe it, and went to check my ruler with a tape measure, which just confirmed my ignorance.
Firstly, I can't understand how, as a very conscientious school child, I came to believe this (and passed my 'O' level maths ok). Secondly, with all the numerous crafts that I do, and the measuring they entail, I can't believe that I have not made this discovery before the age of 57!
Feeling very embarrassed now, and wondering how many other things that I think I know, but don't.