This analogy was shared with us and we share it with you.
Early liquid-cooled engines surrounded their cylinders and heads with lots of water in large water jackets and often had no pump at all. The coolant circulated by convection e.g. hotter water expands, becomes less dense, and consequently slowly rises out of the engine through a really large hose to the radiator where it was cooled, contracting slightly and thus became more dense, falling back to the engine through a second hose at the bottom of the radiator.
This thermosiphon system worked well but as engines became more more powerful the thermosiphon’s slow circulation rate permitted the formation of steam pockets, which resulted in overheated engines.
Early engineers determined that moving the water through the engine fast enough to scour away potential steam pockets which prevented overheating and thus added water pumps. There were still problems so next they began to make the coolant passages smaller. Doing so raised the block pressure and the coolant had to move through them faster.