Because of these properties, professional chefs generally recommend using microwaves for a limited set of tasks, including: melting fats, such as butter, chocolate, cooking grains like oatmeal and grits, thawing frozen meats and vegetables before cooking by other methods, quickly reheating already-cooked foods, and boiling water.
Boiling water does add another danger, however: superheating. In a microwave, water can be raised quickly to a temperature above the boiling point before major bubbles form, especially if it is purified and in a very clean glass vessel. When water in that state is disturbed, it can suddenly and unexpectedly boil violently, leading to a risk of scalding.
Care should be taken when removing heated water from the microwave; make sure that one's hands are protected from possible boil-over, to place the container on a level, heat-proof, surface and to stir liquid with a warm spoon. Also, never add powdered substances (such as instant coffee or cocoa mix) to the container taken from the microwave; it is advised that the water is poured slowly into another container that already contains the powder.
Never put any metal inside a microwave, unless it is part of a specifically microwave-approved product. Electrical arcs may short-cut through the metal, possibly damaging the oven or starting a fire. Even mugs with a bit of metal trim can be dangerous.