Drink it tomorrow morning before the sun rises, while you are sitting on the beach. Your tail will divide and shrink, until it becomes what human beings call ‘pretty legs.’ It will hurt; it will feel as if a sword were going through your body. All who see you will say that you are the most beautiful human child they have ever seen. You will walk more gracefully than any dancer; but every time your foot touches the ground it will feel as though you were walking on knives so sharp that your blood must flow. If you are willing to suffer all this, then I can help you.”
“I will,” whispered the little mermaid, and thought of her prince and how she would win an immortal soul.
The Tales of Hans Christian Andersen, The Little Mermaid.