Espresso is a highly concentrated form of coffee that is served in "shots". It is made by forcing pressurized hot water through very finely ground coffee beans using an espresso machine. The result is a liquid stronger than coffee topped with a "crema," a brown foam that forms when air bubbles combine with the soluble oils of fine-ground coffee and sits on top of a properly pulled shot of espresso. A single shot of espresso requires approximately seven to nine grams of ground coffee and produces one fluid ounce, while a double shot of espresso, called a doppio, requires 14 to 18 grams of coffee and produces two fluid ounces. Espresso is often served in preheated cups with a capacity of 40 ml, containing 25-30 ml of espresso including crema. It is meant to be sipped slowly to take in its full, rich flavor. Espresso can also be used as the base for popular espresso drinks like the Americano, latte, cappuccino, and more.