The metal on a teabag tag or staple does not spark in the microwave because it is relatively small and does not create concentrated areas of high electric field strength.
Microwaves work by generating electromagnetic waves that excite water molecules, causing them to vibrate and produce heat. When metal objects are placed in a microwave, they can reflect and concentrate the microwave energy, leading to the formation of electric arcs or sparks. However, in the case of teabag tags or staples, their size and shape do not typically create areas of high electric field concentration, which prevents sparking.
It's important to note that while small metal objects like teabag tags generally don't cause sparks, larger or irregularly shaped metal objects should not be placed in the microwave, as they can still cause dangerous arcing or damage to the appliance.