Shaking Molecules

Water molecules are polar — they have a positive end and a negative end, like tiny bar magnets.

At a molecular scale, billions of water molecules twist and drag against their neighbors, passing kinetic energy along. The result is heat. Water-rich foods heat especially well, but water is not the only actor: fats, sugars, starches, and dissolved ions can absorb microwave energy too. Your microwave heats food mainly by depositing energy into molecules and ions, then ordinary conduction spreads that heat inward.