How is day caused?

Answer 1

because the rotation of the earth on its axis causes day and night

As the earth rotates, any given point on its surface will gradually turn away from and eventually toward the sun. If you hold a ball several feet from a lit table lamp in an otherwise dark room, you will notice that about one half of the ball is lighted up by the lamp. As you turn the ball (on any axis approximately perpendicular to the line between the lamp and the ball) the portion of the ball which is lit will change, and one complete rotation will bring the same face back into the light. That one complete rotation is the equivalent of one day on earth.