How to make coffee under pressure.
To vastly over-simply things, espresso is coffee made under pressure. Specifically, it’s the result of forcing hot, pressurized water through a layer of finely ground coffee beans in a short period of time—about 20 seconds. It’s this high pressure—ideally nine bars, or nine times the normal level of air pressure at sea level—that produces crema, the tan frothy foam made of coffee oils, air bubbles, and the tail end of the espresso shot.
Home devices like Moka pots, Aeropresses, and countertop espresso makers can make a strong, short cup of coffee similar to espresso, but they lack the power to create sufficient pressure to make a true espresso shot, as evidenced by the lack of crema in the cup. For the real deal, you’ll need to pony up about a thousand dollars to get a proper espresso machine. The crema doesn’t lie.