Density. And amount of water in 2 areas and how much heat losses is there and heat in one spot.
Heated water rise because it is less dense than colder water which sinks, this set up thermsiphon.
Now, you got a cooler Lister and a tank filled with heated water, water will flow to the cooler lister while cold water in the lister sinking and is DRAWN into the warmer tank and got warmer, rises again. So on.
Small pinch of used coffee grind bits in a pot of water, heat pot up on the elements, and watch coffee ground begin to circulate you will see thermsiphon in action.
Cheers, Wizard