Here's why:

* Orbital Position: Both Mercury and Venus orbit closer to the Sun than Earth. This means they can appear between the Earth and the Sun, similar to the Moon's phases.

* Illumination: As these planets orbit, the amount of sunlight reflecting off their surfaces changes from our perspective on Earth. Just like the Moon, we see different amounts of illuminated area, resulting in phases.

Important Note: While Mars, Jupiter, and Saturn are also visible in the night sky, they don't exhibit significant phases due to their much larger distances from the Sun and Earth.