The Moon Phases on a Flat Earth.
It appears that The Moon goes through phases on a Flat Earth Model, because as The Sun and Moon orbit the Flat Earth, The Sun moves faster, than The Moon does. So, it "laps" The Moon every 30 days. This is why the Moon goes through a 30 day cycle of waxing and waning. The Moon appears to be "full" when it is furthest from The Sun; 180 degrees from The Sun on its circuit. The Moon appears to be just a sliver, when it is very close to The Sun.
It order to understand this, you must understand The Moon's source of light. The Moon appears to absorb light from The Sun and then glow in the dark. Like any other glow in the dark object, The Moon will not glow in the daylight. That's why The Moon we see during the day, doesn't glow.
If The Moon was just reflecting light from The Sun, its light would be warm, but it isn't. Even concentrated moonlight, while very bright, is not even warm. Of course, concentrated Sunlight is very hot.