Riddle: Pregnant every time you see her, yet she never will give birth. What is she?
How can one ‘thing’ can be pregnant every time you see her, but never will she give birth? It is only possible if this thing is not an animal or human but something that has been personified with human abilities.
If you think about the round things you see you might find the answer. Besides, as a clue, I can assure you that this thing does not live on the earth but hangs in the sky, yet not always only time to time. Yes, it is the moon, but not any phase of moon it is full moon.
The answer is: Full moon.