When I heat food in my microwave, the food comes out with tough spots and cold spots. Why is that?

Because the microwave energy is beamed into the interior of the microwave and then dispersed by a metallic "stirrer," or by a rotating carousel, the pattern of the energy is usually consistent. However, certain portions of the food may overcook (creating tough spots) and others will undercook. A microwave oven with a carousel is far better at cooking the food consistently. It also helps to use the microwave's preset cooking feature.