They like the sound the plastic makes. Make sure she doesn't eat it because it will be very harmful to her if she ingests it. Just one of her ways of playing. :-) You can buy all the toys in the world for your cat, and the one thing that will keep them entertained is plastic, whether it be a bag, milk cap, plastic stick, string, boxes etc. Stuff that doesn't hardly cost a thing LOL.
Food and water has nothing to do with this. Your cat is just finding things that interest her. Cats are naturally curious, so they will be nosey and explore everything! :-)