Metal objects can indeed get hot when heated in a microwave oven, and in fact, it is generally advised to avoid placing metal objects inside a microwave. The reason for this is that microwaves are a form of electromagnetic radiation that interacts differently with metal compared to other materials.
Metal objects reflect and concentrate the microwave radiation rather than absorbing it. When microwaves encounter a metal object, they induce electric currents called "eddy currents" within the metal. These electric currents generate heat due to the resistance of the metal.
However, the distribution of these currents is uneven, which can cause the metal to heat up rapidly and potentially become extremely hot in certain spots. This uneven heating can lead to sparks, arcing, and even pose a fire hazard. Additionally, the concentrated electric currents can damage the microwave itself.
To ensure safe microwave usage, it's essential to avoid placing metal objects inside the microwave, including aluminum foil, metal utensils, and containers with metallic accents. Instead, opt for microwave-safe materials like glass, ceramic, or microwave-safe plastics that are transparent to microwave radiation and do not pose a risk of sparking or overheating.