Unlike other ovens, which cook food with heat waves made from burning gas or electric currents, microwave ovens use special bands of electromagnetic energy called microwaves (similar to light waves) to cook food. While heat waves gradually work their way inside food to cook it, microwaves can travel right through food in an instant. In a microwave oven a device called a magnetron produces a beam of microwaves that pass through a spinning fan, which sends the waves bouncing in all directions. As they travel through food their energy is absorbed by molecules of water. The water molecules vibrate at the same high speed as the microwaves (2.45 billion times per second!) and rub against other molecules. All this movement and friction causes a great deal of heat, cooking the food inside and out. Microwaved food is cooked through a process similar to steaming, which explains why it doesn’t turn brown. But some microwave ovens have traditional heating elements to make food look more appealing—giving it the outer color that we expect in cooked food. Certain materials allow microwaves to pass through (meaning they are not heated by the waves) while other materials absorb the waves and still others reflect them, or bounce them back. For this reason it is important to be careful about the containers and coverings we use in microwave ovens. Microwaves pass through glass and plastic wrap, for example, which are safe to use, as are paper products and most sturdy plastics. But metal containers and coverings like aluminum foil are reflective. Such surfaces keep food from absorbing microwaves, allowing the waves to bounce around so much inside an oven that it may break.