"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 amongst biochemists is to add small amounts of methylene blue (generally a few drops of a stain solution will suffice) to coffee, cola, or another dark beverage. The stain's color will be masked by the beverage, and its taste is fairly faint. Within a few hours, the methylene blue will have been removed by the prank victim's kidneys, causing his urine to change color. 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, although allergies are possible and it is advisable to use pharmaceutical-grade stain which has been carefully protected from contamination. The main risk of methylene blue pranks is generating a sense of panic in the victim."
haha, i gotta try this