Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it and use it to cook food. Most of the time, it is completely benign. But in large enough quantities, the very same stuff we use to rinse a toothbrush can overturn cars, demolish houses and even kill. Flooding has claimed millions of lives in the last hundred years alone, more than any other weather phenomenon.

FEMA News Photo Heavy rains in the spring of 2001 flooded Davenport, Iowa. Until the waters subsided, locals had to get around town by rowboat.

In this edition of HowStuffWorks, we'll find out what makes water change character so rapidly and see what happens when it does. We'll explore the negative impact of floods as well as some of the benefits. We'll also examine how human construction can contain flooding or, in some cases, cause it.

	During a natural disaster, a battery-operated weather radio could help you stay informed. Click here to look at several different models of weather radios and compare prices.