Here's why:

* The Full Moon: A full moon occurs when the Earth is positioned directly between the Sun and the Moon. This means the entire illuminated side of the Moon faces Earth, making it appear fully illuminated.

However, it's important to remember that:

* The Moon's Orbit: The Moon's orbit around the Earth isn't perfectly circular. It's slightly elliptical, meaning the distance between the Earth and Moon varies.

* Lunar Phases: The different phases of the moon (new moon, crescent, gibbous, full moon, etc.) are caused by the changing angle at which we see the illuminated side of the Moon as it orbits the Earth.

So, even during a full moon, there might be a slight shadow or a very small portion of the Moon that isn't illuminated. This is due to the varying distances and angles, not because a full moon is "impossible."

It's a subtle difference, but it's important to understand that a "completely" full moon is more of a theoretical concept than a reality.