That's a great question! When the Earth moves between the Sun and the Moon, it's called a lunar eclipse.

Here's why:

* Eclipse: The word "eclipse" refers to the blocking of light from one celestial body by another.

* Lunar: This refers to the Moon.

During a lunar eclipse, the Earth's shadow falls on the Moon, blocking sunlight from reaching it. This can make the Moon appear reddish, as only some of the light (longer wavelengths like red) is scattered around Earth's atmosphere and reaches the Moon.