Answer:
The diagram shows the sun and earth, and the x is daytime because it is facing the sun this is because of the rotation of the earth. As the earth turns the sun, beams light to the earth.
for example, take a ball and torch.
put the torch on and put it on the table horizontally next to the ball turn the ball around. You can see that the light hits the ball it becomes day and the back is dark indicating night...
Hope this helps!!