They say that long ago, before things were as they are today, the Moon fell in love with her brother.
She tried everything she could think of to get into his bed, but he was having none of it. Only Cat shared his bed, no one else.
So Moon goes to Cat one day and says: Cat, trade shapes with me.
And Cat, being Cat, says: What's in it for me?
Says Moon: Someday I shall bear a great many children, and my children will always make a place for you at their hearths.
And Cat, being Cat, says: What else?