In chemistry class recently, my teacher was telling us about how a microwave works. She says that the EM waves emitted by the microwave hit the water molecules in food and cause them to vibrate faster by constructive interference with their vibrational motion, and therefore cause an increase in heat energy.
My question on hearing this was, if EM waves heat water by superposition with its vibration, why don't we ever see it cooling water with destructive superposition?
Is my teacher's explanation wrong in some way or am I missing a reason for definite constructive superposition?