If you twirl this ball at night directly in front of a bright light, you will notice that half of the ball is lighted up while the other half is in the shade.That is just like our night and day.

 
  • 如果在夜间正对着明亮的灯光旋转,就会发现半个皮球被照亮,而另外半个在阴暗处,这种情况和我们的白天黑夜的现象相同。
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