Methylene blue is a synthetic dye that has been used for over a century in various applications, including biology and medicine. It appears as a blue powder and is soluble in water. In laboratories, it is often used as a stain to help visualize cells and tissues under a microscope. In medicine, methylene blue has been used to treat conditions like methemoglobinemia, a blood disorder where hemoglobin cannot effectively release oxygen to body tissues. It also has potential applications in treating certain infections and as a diagnostic tool in imaging studies.