What causes a sealed bag of vegetables to expand when heated in a microwave oven?

When a sealed bag of vegetables is heated in a microwave oven, the moisture present within the vegetables turns into steam. As the temperature rises, this steam builds up pressure inside the sealed bag, causing it to expand. If the pressure becomes too great, the bag may burst, releasing the steam and potentially damaging the microwave. Properly venting the bag before heating can help prevent this issue.