A tornado does not necessarily need to be visible; however, the extremely low pressure caused by the high wind speeds and rapid rotation usually causes water vapor in the (21) _____________ to condense into a visible condensation funnel.
Tornadoes can happen in nearly any part of the world. In the United States, a tornado has been (22) _____________ in all states. The middle part of the United States is nicknamed "Tornado Alley" for the large number of tornadoes created there.
A tornado is a tube of (23) _____________ spinning air that touches the ground. Wind inside the tornado spins fast, but the actual "circle" of wind around them is massive. This makes tornadoes very (24) _____________.
Tornadoes mostly happen during strong thunderstorms. They cause a lot of damage to anything in the path. Tornadoes (25) _____________ ranked on the Fujita scale, from F0 to F5. The F0 has the lowest wind speed, and the F5 has the highest one.

The water we drink and use is running short in the world. We all have to learn how to stop wasting our limited water. One of the steps we should take is to find ways of reusing it. Experiments have already been done in this field.
Today in most large cities, fresh water is used only once, then it runs into waste system. But it is possible to pipe the used water to a purifying factory. There it can be filtered and treated with chemicals so that it can be used again, just as it were fresh from a spring.
But even if every large city purified and reused its water, we still would not have enough. Then we could turn to the oceans. All we would have to do to make use of the seawater on earth is to get rid of the salt. This process is called desalination, and it is already in use in many parts of the world.