Abstract: During the past two decades, high-Tc superconductivity has been discovered in cuprates, and colossal magnetoresistance in perovskite manganites; now, Fe, Co and Ni, which used to be considered incompatible with superconductivity, have been found to be superconducting when forming compounds. These systems have completely different physical behaviors, but also have one common characteristic: all the compounds are 3d-electron materials. So, why is it that the same 3d electrons exhibit completely different or even opposite behaviors in different environments? In this paper, the author presents and explores a series of whys of these remarkable phenomena.