[Brian]

Okay, coolant flows from top of radiator to the bottom of the radiator, that gravity thingie don't you know. From there it has to be pumped back to the top of the radiator, via the engine block. "Stuff" don't run up hill. Jeeezze.
Actually Lawson, water does run uphill when its temperature rises...it's known as thermosyphon effect. The waterpumps are only there to assist this process....gravity has nothing to do with it.