Put cold tap water in a saucepan and heat it on the stove. After a few minutes you hear a loud roar coming from the pan. The roar does not mean the water is boiling. Look at the pan and you see bubbles apparently stuck to the bottom. Some of those bubbles are round and steady–they contain oxygen formerly dissolved in the water. The oxygen is coming out of solution because hot water cannot hold as much dissolved oxygen as cold water. Other bubbles on the bottom of the pan appear to vibrate. These bubbles contain not oxygen but water vapor, created from the liquid water by heating from below. As the first vapor bubbles break free from the hot bottom and rise into slightly cooler water a fraction of an inch above, the vapor in each bubble suddenly condenses. The vapor bubble collapses with a loud click.