Why did the Aztecs use cacao beans as money?

Because cacao was a rare luxury that couldn't be grown inside the empire's own borders, so its beans were among the most valuable commodities the Aztecs had. They worked as a fairly low-value form of payment: about 80 to 100 beans could buy a small cloak, or a canoe's worth of fresh water for people living near the salty parts of the lakes around the capital, Tenochtitlan. Money worth having is money worth faking: counterfeiters emptied out real cacao shells and refilled them with dirt or mud, passing them off as the real thing.

Cacao wasn't just currency, though; it was also the ingredient in one of the most prestigious drinks in Aztec society. Chocolate, made by mixing hot water with toasted, ground cacao, maize, chili, honey, and vanilla, was the drink of warriors and nobles, who took pride in avoiding pulque, a common fermented drink, in favor of cacao instead. The drink was treated with real ceremony: beaten with a stick or poured between containers to build up a rich foam, and described by one Spanish chronicler as something 'not drunk unthinkingly.' Rulers were even served specialty versions in different colors, from green chocolate made with tender cacao to black, white, and rose-colored versions flavored with dried flowers and vanilla pods.

Even the ruler Moctezuma II wasn't exempt from austerity: at times he gave up luxuries entirely, swapping his usual chocolate for plain water mixed with parched bean powder. When the Spanish conquistador Hernán Cortés encountered chocolate at Moctezuma's court in 1520, it was this same bitter, spiced drink he saw. It took decades for Europe to acquire the taste: the first official shipment of cacao beans to Europe wasn't recorded until 1585, and when the drink finally caught on there in the 1600s, Europeans had already reinvented it, serving it warm and sweetened, nothing like the bitter, foamy ceremonial drink it started as.