Okay, now, from a chemistry perspective, what is work? Well, when we're in chemistry, we basically want to work with atoms and molecules, and we say that a single molecule or an atom has some kinetic energy, translational energy. And collectively, the translational or kinetic energy of the entire system can do work. But now it doesn't do work by acting over some distance. It does work by expansion. So expansion work is called PD work, and you find the work done by multiplying the pressure when it's constant by the change in volume.