Methylene blue is highly stable in the human body, and if ingested, it resists the acidic environment of the stomach as well as the many hydrolytic enzymes present. It is not significantly metabolized by the liver, and is instead quickly filtered out by the kidneys. A common prank among chemists and biochemists of the early and middle 20th century was to to add small amounts of methylene blue (generally a few drops of a stain solution sufficed) to coffee, cola, or another dark beverage.[citation needed] The stain's color was masked by the beverage, and its taste is fairly faint. Within a few hours, the methylene blue was removed by the prank victim's kidneys, which caused his or her urine to change color[9] The urine may become green if little methylene blue was added; larger amounts create a deep blue color. The prank is fairly harmless if small amounts of methylene blue are used.