The "bump" sound you hear when using a microwave oven is typically caused by the rapid expansion of food or liquid inside a closed container. This phenomenon is known as "superheating" or "superheated liquids."
When water or other liquids are heated in a microwave, they can sometimes become heated beyond their boiling point without actually boiling. This occurs because the surface tension of the liquid prevents the formation of bubbles necessary for boiling. As a result, the liquid appears calm and doesn't exhibit the typical signs of boiling.
However, when the superheated liquid is disturbed or a foreign object is introduced, such as a spoon or a tea bag, it can rapidly boil and release steam. This sudden release of steam creates a surge of pressure that causes the "bump" sound you hear.
It's important to exercise caution when handling heated liquids from a microwave to avoid potential burns. To prevent superheating, you can add a non-metallic object, like a wooden stir stick or microwave-safe tea bag, to the liquid before heating. This promotes the formation of bubbles and reduces the risk of superheating.
Additionally, be mindful of using microwave-safe containers and following the manufacturer's guidelines to ensure safe and proper microwave usage.