why the lid cannot be open after the heating process?

The lid cannot be opened after the heating process due to the buildup of pressure inside the container. As the contents heat up, the steam generated increases the internal pressure, creating a vacuum seal that keeps the lid securely closed. Attempting to open it under these conditions can be dangerous, as the sudden release of pressure may cause hot steam or contents to escape violently. It's important to allow the container to cool down and release pressure safely before opening.