E=MC2 v Pythagoras's Theorem

Despite not owning a jacket with leather elbow patches, my friend is a complete math's geek and for some reason that even now that i don't remember why, we were discussing E=MC2, i assume it was Big Audio Dynamites 80's hit of the same name because i can't see how we would get into it any other way.   
It all took a nasty twist when we went from a song about films to him saying that 'Many people think that E=MC2 is the most history changing equation but it isn't, there is one that had even greater impact'.
Obviously i did what any sane person would do and tried not to encourage by asking 'What's that then?' but he ignored what i assumed was my lack of interest by telling me anyway, Pythagoras's theorem.
According to someone who was oblivious to someone eyes glazing over while franctically searching for a sharp implement to shove in his ear, he explained that this theorem is a fundamental rule in geometry stating that in a right-angled triangle, the area of the square on the hypotenuse (\(c\)) equals the sum of the areas of the squares on the other two sides (\(a\) and \(b\)), expressed as \(a^{2}+b^{2}=c^{2}\) and is used to calculate unknown side lengths, with the hypotenuse being the longest side opposite the \(90^{\circ }\) angle.
Or something, i had lost the will to live by then but managed to squeak out a: 'Oh, that's good then' and willed my phone to ring for an emergency at the Office which would mean i would have to leave IMMEDIATELY!
I cut short some more boring stuff about math's by asking what use is Pythagoras's Theorem in my everyday life and after going through a couple of things which was to do with Construction (I'm not a builder) and Robotics (I'm not a scientist) he hit upon making sure pictures are hung straight.
So there you go kids, stay in school and pay attention in your Math's classes because one day you may look at a painting and see it is everso slightly wonky and you can straighten it and you will have Pythagoras to thank.
Either that or you can nudge it and then say to someone 'Does that look straight to you?' which is what the majority of people do who heard the word Pythagoras at school and thought..ppft...when will I ever need that and looked out the window at the trees instead.