You are correct that putting metal objects in a microwave can cause sparks and potentially damage the appliance. However, some microwaves come with metal racks specifically designed for use in the microwave. These racks are made from materials and designs that prevent sparking and allow for safe use.
The metal racks found in certain microwaves are typically made of materials that do not conduct electricity very well, such as stainless steel or specially coated metal. These materials help to prevent the buildup of electric charges and minimize the risk of sparks.
Additionally, the metal racks are designed with spacing and positioning in mind to ensure that they do not come into direct contact with the interior walls or other parts of the microwave where sparking could occur. The spacing allows for proper airflow and even heating while reducing the chances of arcing.
It's important to note that not all microwaves are equipped with metal racks, and using metal objects or cookware that are not specifically designed for microwave use can be dangerous. Always refer to the manufacturer's instructions and guidelines for your specific microwave model to ensure safe and proper usage.