This afternoon after lunch, in my cubicle I overheard my labmate and another student, heads down hunched over a pad of green engineering paper, busily working through a basic mechanics problem - and by the end of the conversation, they had derived from basic tensor theory the deformation expected in an infinite plane of compressed material, as opposed to an axially loaded rectangular bar, which requires a correction using the bulk modulus because Poisson's Ratio is not in effect.
Whew. Let it be known that I can't derive this from my head like my labmate did - there would be heavy textbook referencing for me to get there... and maybe some Wikipedia entries...
At the end of the whole derivation, however, the answer was a fairly standard equation, and any mechanics folks would recognize the method.
"Do I have to cite this in my conference paper?" asked the thoroughly dazzled student.
"No," says my labmate, "That would be like citing Hooke's Law... it's a commonly understood thing that uses a trivial derivation to those in the field."
There was a moment of silence in the office, and then a small voice piped up from the other end of the room.... "Um, I just cited Hooke's Law in my paper..."
Some of us are more thorough than others!