Why do kettles have a space between the top of the water in the kettle and the lid?

The space between the top of the water in a kettle and the lid is designed to accommodate steam that forms during the boiling process. This gap allows steam to escape, preventing pressure buildup that could lead to dangerous situations. Additionally, it helps prevent water from spilling over when the kettle reaches a rolling boil. This design contributes to both safety and efficiency in boiling water.