system
Given the following post, respond as if you're commenting on Reddit:
human
Microwave ovens work by exciting water molecules in food. What other substances can be excited by a microwave oven?
gpt
Many substances can be heated in a microwave oven. The mechanism behind microwave ovens is called *dielectric heating* and it requires what are called *polar molecules*. A polar molecule is a molecule where the distribution of positive and negative charges isn't the same and there are parts of the molecule that have a net positive or negative charge.

A polar molecule, like other molecules, is still neutral as a whole, but it has a distinct positive and negative part.

When a polar molecule is exposed to an electromagnetic field, the molecule will rotate to align itself to the field. If the field oscillates, the molecule will constantly swap between rotating back and forth. While it's doing that, it will repeatedly push and bump nearby molecules and thereby increasing the average kinetic energy of the molecules in the substance (and thus the temperature).

This mechanism works with all polar molecules. Water is a polar molecule, but so are fats and sugars. The effectiveness of the microwave oven depends on how skewed the distribution of negative and positive charge is in the molecule. This is quantified as the *dipole moment* of the molecule. A molecule with a greater dipole moment will react more strongly to the microwaves than a molecule with a small dipole moment.

Note that, unlike what some people assume, microwave ovens aren't configured to a specific frequency that makes them work on water. The process of dielectric heating works with electromagnetic radiation of various frequencies. And even microwave ovens don't all use the same frequency. Consumer models typically use 2.4 GHz, whereas industrial devices operate at higher frequencies (10 GHz).