Understanding marine pumps begins with grasping how liquids behave in an enclosed space. For starters, liquids behave fundamentally differently from, say, the gases in your engine’s cylinders. Gases can be compressed; their density changes when the pressure on them changes. But liquids cannot be compressed. Under pressure, a liquid’s density remains constant. Because liquid doesn’t compress, the only other thing it can do when it’s placed under pressure is to expand. If there’s an open space in the system, the liquid will expand to fill that space, and even spill out from it. When water runs through a pump’s outlet port or out the tap of a galley faucet, it’s doing exactly what it’s designed to do. But in some cases—say, at the overboard vent to a fuel tank—you might see an undesirable, even illegal, result of the same principle. If you overfill the tank, what you’ll see is fuel spilling out through the vent and into the harbor. In either of those cases, desirable and undesirable, the liquid expands till it leaves the system.