Choose Your Poison by ladydove7
"I will prepare a draught for you, with which you must swim to land tomorrow before sunrise, and sit down on the shore and drink it. Your tail will then disappear, and shrink up into what mankind calls legs, and you will feel great pain, as if a sword were passing through you."
"But think again," said the witch; "for when once your shape has become like a human being, you can no more be a mermaid."
-From Hans Christian Anderson's "The Little Mermaid"