Magic World-E. Nesbit

The Light Princess is a fairytale written by George MacDonald about a young princess who is cursed by her wicked aunt to be without gravity. The only time the princess is not light and able to be normal is when she is in water. She soon grows an attachment to the lake near her palace and spends as much time in this lake as she can. A young prince who set out to find a princess to marry stumbled upon this young princess' palace. She was nothing like any princess that he had met before, and they soon spent nights swimming together in her lake. But when the lake starts to dry up, due to the same witch who removed the weight from the princess, the princess starts to wither away with it. In order for the lake, and consequently the princess, to be saved, someone has to plug the hole that the water is escaping through, and therefore sacrifice their own life. The prince eagerly volunteers to give his life to save the princess. Despite the odds, he saves the lake, the princess, and manages to survive as well. The lake becomes full again and the princess regains her gravity. All is well in the kingdom and the prince and princess live happily ever after.