So we see that in a heat engine, when the piston is moving, the force is changing, and so is pressure, and so is volume, but temperature remains the same. Okay, so what this basically means, or what this basically implies, is that heat must be converted to work, and all the heat must be converted to work. But that's actually not true, and only about 10% to 20% normally is converted to work. Okay, and let's see why. Well, let's go back here. When the energy is transferred into this system, this system, the volume increases because the piston starts moving this way, and it continues moving this way until when?