Moka Pots

How do they work?

The Moka pot has 3 parts that are screwed together tightly:
moka pot cross-section
When the water in the bottom is heated up it generates steam. This steam increases the pressure in the bottom chamber and this pressure starts to push the water up through the filter and into the top compartment.
moka pot cross-section: water rising through the filter
When the rising water reaches the ground coffee, the coffee soaks some of the water and expands - so it effectively tamps itself. The tamped coffee puts up a resistance that the rising water has to overcome - so as a result higher pressure is needed to push the water up (that's why we call this an espresso maker rather than a coffee maker).
moka pot cross-section: brewing
When the last bit of water is going up to the top chamber the Moka pot makes a characteristic gurgling sound and the brewed coffee at the top is ready for drinking.
moka pot cross-section: finished

Further Reading

Types of Stovetop Espresso Pots
Stove Top Espresso Maker: Aluminum or Stainless Steel?
Stainless Steel Stovetop Espresso Maker
Stove Top Espresso Coffee Maker: Tips