An uncovered bowl of soup can "explode" in the microwave before the soup boils due to a phenomenon known as "superheating." Superheating occurs when a liquid is heated above its boiling point without forming bubbles. This can happen in very smooth and clean containers, such as a clean microwave-safe bowl, where there are minimal nucleation sites (points where bubbles can form).
When you heat a bowl of soup in the microwave, the water molecules absorb the microwave energy and start to heat up. However, without nucleation sites, the water may not form bubbles and can exceed its boiling point without boiling. This situation is unstable and highly sensitive to disturbances.
If you were to take the superheated bowl of soup out of the microwave and accidentally jostle or disturb it, or even place a spoon in it, it can suddenly start to boil violently, leading to an explosion of hot liquid. This sudden boiling is called a "bump."
To prevent this from happening, it's a good idea to cover the bowl with a microwave-safe lid or use a microwave-safe stirring rod (e.g., a wooden chopstick) while heating the soup. Creating a few nucleation sites can prevent superheating and reduce the risk of the soup exploding when disturbed. Additionally, using a microwave-safe container with rougher surfaces can also help reduce the likelihood of superheating.