Going back to the famous E=MC^2 equation, “I was once told” (no cite) that every action involving energy meant some small conversion of energy to/from mass was going on.
I took this to mean ; If I light a match, the light/heat energy comes from a chemical reaction (burning) but that involves a small amount of mass in the match being converted to energy.
Or, if I compress a spring (adding potential energy) somewhere (in the spring? or muscles in my body?) a small amount of mass was being converted to energy.
Is that so?
FWIW - I always had the simplistic view that if, say, a ball rolled down a hill it was merely converting one form of energy (potential) to another (kinetic). Is far more going on?