Earth and Sky科普英語：Water Cycle 水循環(huán)

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All the oceans on earth are salty, while inland water is typically fresh. How does water become salty? We'll answer a listener's question -- today on Earth and Sky.

DB: This is Earth and Sky. We recently visited the Austin Children's Museum in Texas to record your science questions. This one is from Jeana Lungwitz.

Jeana Lungwitz: Why are the oceans salt water and the rivers and lakes typically fresh water?

JB: Jeana, first imagine a tea kettle. If you boil tap water to make tea, some of it evaporates. After a while, you might notice a hard crust of mineral deposits forms on the bottom of your teapot

DB: That crust is there because when tap water evaporates it leaves behind any dissolved salts and minerals. Now take this scenario to the ocean. Most water on Earth evaporates from the surface of the ocean. There it leaves behind dissolved salts. Then, when water precipitates out of the atmosphere, it falls as fresh water.

JB: The freshwater that falls onto land cascades over surfaces like rocks and sediments. Here it picks up nutrients, chemicals, and salts. The water carries the dissolved salts on its long journey back to sea. If you run this water cycle over and over again, through four and a half billion years, you eventually leave a lot of salt in the ocean.

DB: In fact, if you came back in another billion years, the ocean would be even saltier than it is now. Thanks for your question Jeana. And with thanks to the National Science Foundation, we're Block and Byrd for Earth and Sky