A microwave oven does emit thermal radiation to heat up food. Microwave radiation is thermal radiation. For some reason, pre-college teachers and books have a mistaken notion that thermal radiation = infrared radiation. All frequencies of the electromagnetic spectrum carry energy, from radio waves, microwaves, infrared waves, visible light, ultraviolet, and X-rays to gamma rays. All frequencies of radiation heat up an object that they strike and therefore can be thermal radiation. When physicists use the term "thermal radiation", they either mean radiation that has the ability to heat up an object it strikes. Or they mean a broad spectrum of frequencies with a certain shape that depends on the emitter's temperature.