Since the 1920s, the formulas of mass and gravitation have worked perfectly with high accuracy. However, the basic principle of these two phenomena remains unknown. What is the origin of mass? How can spacetime be curved by mass? What is the mechanism by which spacetime creates gravitation? . . . The solution to these enigmas lies in general relativity. A thorough examination of the Einstein Field Equations highlights some minor inconsistencies concerning the sign and the meaning of tensors. Solving these inconsistencies fully explains the curvature of spacetime, mass, and gravitation, without modifying the mathematics of general relativity. Moreover, this explanation shows that mass and gravitation are two similar phenomena that can be unified in a common 4D Higgs compatible theory. Applications to astrophysics are also very important: black holes, dark matter, dark energy. . .