Why does the moon change its shape every day?

What we see as light from the moon is actually a reflection of light from the sun. The shape of the moon we see depends on the angle formed between the sun, the earth, and the moon.

For example, when these 3 objects are aligned in a straight line, in the order of sun, earth, moon, we will observe a full moon, where the moon is a full circle. This is because all of the light from the sun is reflected by the moon, without any blockage from the earth. If these were aligned in the order of sun, moon, earth, however, you would not be able to see the moon anymore, since the moon itself is blocking the sunlight and the light is reflected back to the sun, making it invisible from the earth. As the moon orbits between these two positions, you will see various crescent and half shapes from the moon, depending on the position of the moon relative to the earth and the sun.