The witch willingly helps her by selling her a potion that gives her legs in exchange for her beautiful voice, and warns that once she becomes a human, she will never be able to return to the sea.
The witch willingly helps her by selling her a potion that gives her legs in exchange for her tongue (as the Little Mermaid has the most enchanting and beautiful voice in the world). The Sea Witch warns that once she becomes a human, she will never be able to return to the sea.