The heart is the core of all sentient objects in the game. It is a physical object and it is the only object in the game that can generate energy.
A heart is a physical object. The mass of a heart determines how much energy it generates per second. There is no concept of storing the energy a heart produces in any form of container. The energy a heart generates over a period of time (like one second) is either utilized or it is lost forever.
A heart is a growable object.