A cup of water is an item made by using an empty cup on a water source, either sink or fountain. Cups of water cannot be heated using a fire or range, and are thus useless in the creation of the Guthix rest potion. Furthermore, you cannot add nettles to this item - though you can add them to a Bowl of water, and add Nettle tea to an empty cup. Humidify cannot be used to create cups of water from empty cups.
A cup of water cannot be used to counter the desert heat effect because the water in the cup evaporates under the heat.
