God looks down and notices that Adam is all alone while all the animals have companions so he decides to create a companion for man as well.
He comes to see Adam and says to him, "Adam, you are my greatest creation and therefore, I am going to create for you the ultimate companion. She will worship the very ground you walk on, she will long for you and no other, she will wait on you hand and foot and obey your every command, and all it will cost you is an arm and a leg."
Thinking for a few moments, Adam replies reluctantly, "I don't know God, an arm and a leg is an awful lot, what could I get for, say, a rib..."