When you put a kettle full of water on the fire and the water boils, you will see bubbles rising to the surface. That is because when the fire makes the kettle hot, the heat makes the air that is in the water expand, and the air escapes to the top in bubbles. These bubbles are little bags of water with air inside. These bags of hot air are made at the bottom of the kettle. Then they rise till the surface of the water is one mass of bubbles.