Next we take the same fax paper setup which would give us reproducible patterns and place a large steel ball on top of the whole setup. The ball reflected the microwaves around and changed the whole pattern. So, the pattern of hot and cold spots depends on what you put in the microwave. This is why microwave ovens have rotating tables in them. If one can never predict where the hot spots are, then one has to move the food around to make sure that on average all parts of the food see hot and cold spots alike.
Some microwaves have a little metal 'ceiling fan' in front of the microwave emitter.
It is supposed to spin around and reflect the microwaves around so your food can sit still. Note that it was not at all spectacular to put metal in the microwave in this case. Metal simply acts as a mirror for microwaves. The surface of the ball may heat up a little as it subjected to microwaves, but it has so much mass to absorb this energy. The surface of the ball might heat up a little from the microwaves, but there is so much metal inside to soak up the heat that the ball will not even feel warm to the touch.