Day time is when you can see the sun from where you are, and its light and heat can reach you. Night time is when the sun is on the other side of the Earth from you, and its light and heat don't get to you. We get day and night because the Earth is turning and sometimes you can see the sun, and sometimes you can't.
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About day and night
The sun lies at the heart of the solar system, where it is by far the largest object. It holds 99.8 percent of the solar system's mass and is roughly 109 times the diameter of the Earth — about one million Earths could fit inside the sun. ... The sun is one of more than 100 billion stars in the Milky Way.