19 - Populations
About 70% of the Earth’s surface is covered in water, and almost all of that water has been around since the Earth formed billions and billions of years ago. That means a glass of water you drink today could be water that a dinosaur once sipped. Water is constantly recycled on Earth as rain, snow, sleet and hail. It makes its way in and out of oceans, lakes, streams, hail, and glaciers. Scientists call the recycling of water the water cycle (not that bad, huh?).