Microwave ovens primarily heat food through the process of dielectric heating, which is the absorption of microwave energy by water molecules in the food. However, it's important to note that microwave ovens can still heat certain food materials that do not contain water, although less efficiently.
While water is the primary molecule that absorbs microwave energy, other substances with polar molecules, such as fats and sugars, can also absorb some microwave energy and generate heat. These substances can become hot in a microwave oven and indirectly transfer heat to surrounding food components.
However, materials that lack polar molecules, such as most plastics, glass, and ceramics, generally do not absorb microwave energy and therefore do not heat up significantly in a microwave oven. Microwave-safe containers are designed to allow microwaves to pass through without absorbing energy, thus preventing them from getting hot.
It's worth noting that even if a food material doesn't contain water, it can still indirectly heat up in a microwave oven if it is in contact with or surrounded by a water-containing substance. The water molecules in the adjacent material will absorb the microwave energy and transfer heat to the neighboring food component.
In summary, while water is the primary molecule that absorbs microwave energy, certain food materials without water content can still heat up in a microwave oven, especially when in contact with water-containing substances.