If you look at the moon tonight, you'll see something like the picture above. The phase is a cresecent moon. The moon is lit, of course, by the light from the sun. We see the light that has traveled from the sun, reflected off the moon and traveled to the earth.

So why are there two levels of brightness seen in this picture? The answer is called planetshine (and since we're talking about the earth it's called earthshine). The darker light is from light that has traveled from the sun to the earth, reflected off the earth to the moon and back to the earth.

So, amazingly, the dim light you see has traveled from the sun, to the earth, to the moon and back to the earth. Pretty cool if you ask me.