tuffnut:  well missy, i can only commend you on the huge effort you have put into your profile. you will be glad to know it is having your desired effect on me !!!
i'm not a smart bloke, but i can work with fractions without a calculator ;-)
what's your favourite formula ?

missy: LOL!. Excellent question tuffnut, you know how to differentiate yourself. And very clever way to appeal to me. Well done. I'm impressed. What's my favourite formula?. Do you expect me to say Einstien theory of relativity - because that would be on the top of everyone's list because it is so accessible?. That formula impresses me due to the simplicity of the execution, and the complexity of the concept. Like children, I feel you musn't have a favourite because they are all beautiful if they lead to understanding that did not exist before the formula. But my favourite concept is Schrodingers Cat, because it pushes you to grasp how many levels that concept can work on. :) x