Sal Khan
What is the Earth made up of and how do we know? What causes the seasons? Are there longer-ranging cycles in Earth's climate? Source: Khan Academy (www.khanacademy.org), used under the Creative Commons Attribution-NonCommercial-ShareAlike (CC BY-NC-SA) license. All Khan Academy content is available for free at www.khanacademy.org. Khan Academy does not endorse the World Public Library.
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