Explain why it is that we have periods of night and periods of day.?

Explain why it is that we have periods of night and periods of day.?
because the earth rotates on its axis?
I'm not sure because the question asks about periods in the Woman's Health section.
Because the Earth makes one complete spin on its axis every 24-hours. And since the Earth is round, only one side of it faces the sun at any given time while it spins on its axis.
periods are periods. period.always smelly and stinky and dirty with a spot on the back side
That's the way the universe was made. It was explained in elementary school.