<<Somethings yo want to be flexable , like your knee joints , some things you want to be stiff like the foundations of a large building.>>
Your metaphor sucks. The Constitution is neither a knee joint nor a large building. If your knee joint were totally flexible, BTW, your legs probably would not be able to bear your full body weight. Similarly with building foundations - - make them too rigid and they are less able to withstand seismic shocks.
<<Which quality do we want for the constitution?>>
I would say, unless you have figured out a way to return modern society to an exact replica of 18th century America and keep it there, you definitely would want a reasonably flexible Constitution.