The rate of energy absorption by water molecules in relation to frequency. The best transfer of energy occurs at wavelengths of 1-3cm and fall off rapidly afterward.
Microwave ovens operate at a "de-tuned" 12cm to allow the waves to penetrate further into and through the food, otherwise all of the energy would be absorbed in the first layer of water encountered. Your hamburger would be jerky dust on the outside but still bloody on the inside.
Cool!
I wonder if the military (or the Evil Dr. No) has thought about focused battlefield microwave-cook-your-enemy devices??