The positive mass theorem is one of the milestone results in general relativity. Both the proof of Schoen-Yau and that of Witten heavily relied on sophisticated analysis of PDEs on manifolds. There appears to be a prevalent pursuit among physicists to seek a physical understanding to the positive mass. This talk seems to be of this nature. Following the ideas of Roger Penrose, we reduce this problem to a spacetime causality comparison and the negative mass is ruled out by using the focusing theorem. It provides an elementary non-PDE viewpoint to understand the positive mass. This talk is based on a joint work with Xiaokai He and Xiaoning Wu.