What have you learnt today?

[Gilrandir]

As for your link, too many formulas, I can deal with explanations but formulas freak me out, I do not always find the transformation from model to formula easily understandable either, it could be wrong for all I know.

There is a joke: One student is reading a book. Another comes and asks: "What are you reading?" The first replies: "This is quantum physics and I have an exam in it tomorrow". The second says: "But you are holding the book upside down". The first goes: "It doesn't really matter".