Rainwater is water that falls from the sky as rain and can be collected before it soaks into the ground or flows into streams and rivers. When rain hits your roof and runs through gutters into a barrel, that's rainwater you've captured. When puddles form on the sidewalk during a storm, that's rainwater pooling on the surface.

People have collected rainwater for thousands of years, using it for drinking, watering gardens, and washing. In many parts of the world today, families still catch rainwater in large tanks because it's clean, free, and doesn't strain local water supplies. Even in cities with modern plumbing, some people collect rainwater to water their plants or wash their cars.

Rainwater is surprisingly pure when it first falls, though it can pick up whatever it touches on the way down. Rain landing on a metal roof stays cleaner than rain running across a dirty driveway. Ancient civilizations built elaborate systems of channels and cisterns to capture and store rainwater, and some of those systems still work today.