Espresso is not a type of bean, but a brewing method. The word 'espresso' means 'pressed out' in Italian. The first modern espresso machine was patented by Luigi Bezzera in 1901. The science of espresso relies on 'pressure-driven' extraction. Hot water ($90-96$°C) is forced through finely-ground coffee at 9 bars of pressure ($130$ psi). This high pressure extracts more oils and solids than gravity-based brewing, resulting in a concentrated shot. A key feature of a good espresso is the 'crema'—the reddish-brown foam on top. Crema is an emulsion of coffee oils and carbon dioxide bubbles, and its presence indicates the freshness of the beans and the quality of the extraction.