Turkish street coffee uses a sand-filled pan: the pot, called cezve, has coffee and ground coffee inside. It 'magically' fills up almost immediately because the pan is very hot.
I feel stupid for asking but I googled and the "simple" explanation I found confused me even more.
it's "filling up" because the water is boiling. the sand surrounds the vessel on all sides except the top, and it's not a very big pot.