Caroline's Ponytail

Caroline's ponytail (also known as Caroline's bracelet) was first seen in "Magic Potion". Is what allows Caroline to be a mermaid.

Magic
Before the series began, it was just an ordinary ponytail. When Caroline threw the potion in the sink the ponytail accidentally fell in and got wet with the potion. When she showed David that she was a mermaid, her transformation didn't work. It was later found out that Caroline can only turn into a mermaid with the ponytail. She usually wears it as a bracelet. In season 2, the sirens stole her ponytail and she changed it to another one. Her ponytail has been seen in all episodes.