Final answer:
Arching is the spark that occurs when microwaves react to metallic substances inside the oven. It can damage the oven or cause a fire.
Step-by-step explanation:
When microwaves react to metallic substances inside the oven, a phenomenon called arching occurs. Arching is the spark that can be seen inside the microwave and is caused by the interaction between the microwaves and the electrons in the metal.
When the microwaves hit the metal, they create a strong electric field that causes the electrons to move rapidly. This movement of electrons creates sparks, which can damage the oven or even start a fire.
To prevent arching, it is important to avoid putting any metal objects inside a microwave oven.