In one of my stories there is a world where the population believes that the moon is a dragon's egg, and that a new egg is laid each night (or day, depending on the phase). In this fictional mythology, the world is flat and the sun is a fireball that's belched forth by another dragon every morning. For these reasons, the moon cannot be illuminated by the sun as it is for our world. My question is, how would a believer in this mythology explain the phases of the moon?
I figured that the mother dragon could go through monthly hormonal cycles that would produce brighter and darker eggs (also, possibly baby dragons of lighter influence and baby dragons of darker influence), but what would actually create the light in crescent shapes? I figured maybe some kind of luminous fluid inside the eggs, but it seems odd that it would be consistently in that shape.
Any ideas or thoughts on the matter?