The Moon doesn’t change shapes. It’s always a round ball shape, just like the Earth is. What’s changing is how much of the Moon we can see each night. It doesn’t give off its own light, like the Sun. Instead, it reflects the Sun’s light back to us on Earth.
If the Moon were always in the same place, we’d see it pretty much the same way every night, but it’s not. It travels around the Earth, just like the Earth travels around the Sun. Also, just like the Earth, sunlight only lights up one side of the Moon at any time.
If the entire side of the Moon that is lit up by the Sun is facing us, we see a full moon. If only half of the Moon’s lit-up side is facing us, we see what’s called a “quarter moon,” which looks like half a circle. Some nights there’s no moon in the sky at all. Other nights there’s only a sliver. It all depends on how much of the Moon’s lit-up side we can see from Earth.