Thousands of years ago, people used wind to sail boats. Hundreds of years ago, the first windmills were being used in Persia and China. They were used to grind grain. Later, windmills were used to pump water. They were used to irrigate crops. They were used to drain flooded land.
Windmills are machines. Windmills change wind energy into mechanical energy. Even today they are used to do the same jobs or even to make electricity. But where does the energy of wind come from?
It all starts with the sun. The sun sends energy to Earth in the forms of heat and light. The sun's energy heats the Earth. More of the sun's energy reaches the middle parts of the Earth. Less of it reaches the Earth's north and south poles. When the sun shines on land, it heats the land. When the sun shines on the Earth's oceans, it heats them, too. Land heats up quicker than water. Water holds the heat longer.Paragraphs 4 to 5:
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