I gotta delete some stuff tomorrow...
5.56mm
I had a buddy who had an interesting theory along those lines. When people commented on his thinning hair, he would patiently explain that he's acquired so much knowledge that his head has expanded and the hair has to spread out over a larger area giving the illusion of hair loss.
The explanation seemed perfectly reasonable to all of us middle-aged guys there. When his children and grandchildren objected, it was quickly decided by us old guys that their brains just hadn't expanded enough to grasp complex theories.