This is something that's bugged me since I've read about it. What does it mean to say "energy is the capacity to do work"? I've read somewhere that work is some sort of integral. But I'm still confused, what do we mean when we say "the energy of object x is so and so"? I know the two quantities are related by the work-energy theorem, but I think there's a difference between knowing that such an equation exists and actually understanding it.