If you multiply this side by distance, we get pressure times area times distance. But remember, that area times distance is simply the volume. That is, if we go back to our Piston example and the Piston moves a certain distance, say, D, then this area A times distance D will give us this whole volume here or the change in volume. Therefore, pressure times change in volume is equal to work. And that's how we derive this equation, from pressure. Okay?