If you live in a big city, you may see these dotting the rooftops of many buildings. If you live in a smaller town, you may see them towering over neighborhoods or along back roads in the countryside. What are we talking about? Water towers, of course!

Have you ever wondered why water towers exist? Are they really full of water? Why does anyone need a tower of water? Does it have a spigot so you can drink from it like a water fountain?

Water towers are indeed full of water. And, no, you can't drink from them like a water fountain. Every water system needs water towers to provide water in the case of an emergency.

You've probably noticed that other utilities may go out from time to time. In a storm, you may lose electric power. Your television or telephone service might also experience interruptions occasionally. But what about your water?

Do you remember a time recently when you weren't able to turn on the faucet to get water? If you can't, that's great. Most water systems are very reliable, and water towers play an important role in providing that reliability.

Water towers come in all sorts of shapes and sizes. However, they all work the same way. Water towers are nothing more than simple elevated tanks of water.

In case of emergency, water systems need reserves of water, so that people can continue to have access to water while a problem with the water system if fixed. So why don't water systems just build big ponds or use underground tanks? Water pressure, that's why!

When you turn on your water faucet at home, the water comes out because the water is under pressure. Your local water system pushes the water to your home. A typical water system supplies water at a pressure of somewhere between 50 and 100 PSI (pounds per square inch).

Water towers are tall and are often placed on high ground, so that they can provide sufficient pressure to deliver water to homes in case of an emergency. Scientists estimate that each foot of a water tower's height provides a little less than half a pound per square inch of pressure.

Experts can use mathematical formulas to determine how tall a water tower needs to be to provide the right amount of water pressure for all the homes and businesses in a particular area. In areas with lots of hills, water towers can be placed on higher ground, which means they don't have to be as tall. In flat areas, taller water towers may be necessary.

So how much water can most water towers hold? A lot! An average backyard in-ground swimming pool will hold over 20,000 gallons of water. Most water towers can hold 50 times that amount -- that's one million gallons! The exact size of a water tower tank can vary greatly, but most are built to hold approximately one day's water supply.

Water towers can be unique symbols of local pride. Sometimes they are painted with a town's name or a local school's . They can also take on interesting shapes, such as this giant peach in South Carolina.

As for the smaller water towers you see on the roofs of buildings in large cities, those are often required by local laws, so that buildings above a certain height will have their own sources of water in an emergency. If you ever visit New York City, you can see hundreds of buildings with their own water towers.

#### Wonder What's Next?

Tomorrow’s Wonder of the Day unravels a mystery that’ll really give you something to chew on!