Drinking a strong cup of joe on an empty stomach can be a dangerous game. For those looking to avoid a bout of acid reflux, vendors in the Indonesian city of Yogyakarta claim they can neutralize coffee’s acidity with a unique trick. They plunk a piece of red-hot charcoal straight into glasses of the sweetened, black brew.

This is charcoal coffee, known in Indonesia as kopi (“coffee”) joss. Its name comes from the sizzling sound a burning lump of charcoal makes upon hitting the liquid. Some say the bubbling beverage tastes like regular coffee, while others describe a distinct caramel-y, burnt sugar taste.

Yogyakarta (also called Jogja) is located on the island that gives coffee its nickname: Java. Dutch colonists introduced the coffee plant to the Indonesian territory in the 17th century, and it came to dominate global production. By the time the island became a popular tourist destination in the 20th century, Java had become synonymous with its signature export.

In the 1960s, a local coffee-stall owner differentiated his product by using lit charcoal. According to legend, his name was simply “Mr. Man.” Man added the charcoal to a cup of hot water, coffee grounds, and sugar, then left the drink to gurgle and hiss. Once he removed the cooled nugget, customers drank up. Initially, Man’s fans were relegated to young thrill-seekers, but after (medically unsubstantiated) praise for the charcoal coffee’s health benefits spread, kopi joss gained a wider audience.

Today, several copycat stands operate in the surrounding area. Sellers still tout the drink’s ability to alleviate gastrointestinal discomfort, but most customers are attracted to the sheer novelty of watching hot charcoal force a tall glass of coffee to boil over.