The Curvature of Spacetime Is on the Left Hand Side of Einstein's Equation

Janus Funk says that all kinds of energy, including things like pressure and stress, contribute to the curvature of spacetime. The only physical effect in good old classical Newtonian mechanics is differences in potential energy. In general relativity, it's a little bit different because the total amount of energy does matter. But there's another subtlety on the right hand side of Einstein's equation which tells you the source for gravity.