After thinking about E=MC^2, I thought that mass is the only store of energy. I have found many “stores of energy” that are actually just increase in mass. For example kinetic energy, it’s not a store of energy by itself, it’s actually because of special relativity that it has more mass. When something is hotter it doesn’t store energy by temperature, it has energy because the particles are vibrating faster and so have more kinetic energy, therefore having more mass due to special relativity. I just want to ask is it true for all stores of energy that it’s just a increase in mass somehow.