In a microwave oven, the frequency is chosen to match the resonant frequency of:

1. Food heats because something inside it absorbs the waves. 2. The absorber is the most common molecule in food. 3. The frequency is selected to match the resonant frequency of water molecules. 4. Energy then transfers efficiently to the kinetic energy of the molecules. _Source: NCERT Class XII Physics Part I, Ch 8 'Electromagnetic Waves'_