The external application of mannitol should not be mixed with water. It is crystalline and has the effect of absorbing water and dispersing knots, so it should not be mixed with water. Crush it, wrap it in gauze and put it on the affected area, not too thick, to stick to the skin, and replace it after the mannitol clumps. If the gauze falls off, avoid picking it up and using it again, you need to replace it with new gauze and repeat the above method to avoid any infection. In case of infection, you need to go to the hospital promptly and be treated under the guidance of a doctor.