There is a rubber black piece(or metal piece which according to different mist maker models) on the mist maker that is a low-water sensor. It must be almost completely submerged to operate. The optimum operating depth is between 60mm to 80 mm. The mist maker must be kept near the surface of the water for proper operation. The included float will help keep the Mist Maker at the proper level.If u still can't understand ,we suggest you buy the float together!