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。