Even carbon dioxide can dissolve in the groundwater, causing the water to be naturally carbonated. This water is sometimes sold as mineral water. Groundwater can be heated by magma below the Earth's surface. The heated water can create hot springs, springs with water that is naturally hot (Figure 13.23). Some hot springs are used as natural hot tubs and are considered therapeutic and spiritual by some people. However, hot springs can be dangerous, too.
We used a level tablespoon of salt in a cup of water. The batteries we have shown here are more correctly called cells. An actual battery is made up of two or more cells. The zinc-air cell produces about three quarters of a volt.
What that battery is made of, how it works, and how to keep it working well. That phone in your pocket is a modern miracle. Ditto the laptop on your desk, the tablet in your backpack, maybe even the watch on your wrist. And regardless of what each is capable of, they all have one cornerstone component to thank, one that you probably ought to know how to take care of: A battery.