Mercury is Earth's sister. Mercury's concerned for Earth, because she's sick. There are tiny creatures on Earth that she calls humans. They do things like fight each other, build buildings, chop down trees, and other things. There are other creatures that live on Earth, but they don't hurt her (but the humans do, and they're why she's sick). Earth is covered in land and water, while Mercury doesn't have much water. She cares a lot about Earth, and thinks of herself as the 'fun sister'.