 So at a microscopic level, heat energy is really just the kinetic energy of the particles moving around. So if you have a gas of particles and they're all moving around at different velocities, then they each have different kinetic energy, but on average you'll have a certain amount of energy stored in the kinetic energy of that gas, and that's the heat energy ultimately. And the hotter something is, then all these things are bouncing around and they'll have this bigger, hotter distribution of energy, they'll have more energy per particle on average. So in the context of trying to get energy out of our muscles and to do useful things, then normally we think of the work as the important thing, so this is stuff done by the force that our muscles produce, and we think of the heat as a waste byproduct. And indeed, that's often the case. So often when you're trying to use energy, you end up producing some kind of low-temperature heat as a result. And if you produce high-temperature heat, like if you're building a fire in a steam engine, you can use that to do work. But if you have low-temperature heat, then you're basically, that energy is not in a usable form anymore. And there's this whole discipline of physics called thermodynamics, where we talk about exactly what the limits are on trying to extract energy out of different sources of heat. So when you talk about using energy, you're really talking about transforming energy from one form to another form, or sometimes from one form to multiple other forms, of which you really only want one of them. And that means that we always have to worry about the efficiency of that process, which is just the fraction of energy that you extracted into the form you wanted divided by the total amount of energy that you used. So in this case, it would be the work divided by the chemical potential energy that we used.