Thought everyone knew that?

It seems yesterday's post raised as many questions as answers for some people.  So - a couple of explanations.  Firstly, a prime number is one which cannot be exactly divided by any other number - strictly, cannot be exactly divided by any number other than itself and 1.  So 5 is prime for example, but 6 is not, because it can be divided by 2 and 3.  And secondly QED stands for quod erat demonstrandum which translates as "which was to be demonstrated" and is what you put (or we used to put when I was at school) when you've come up with a proof of something - sort of "so there you go then".