Answer:
Sally, Lisa, and Bernadette are triplets. But Sally and Lisa share something that Bernandette does not. What is it?
You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
Humans have two that shrink in the light and expand in the dark
It is a form of plastic money.
Its days are numbered.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?