Now I'm thirsty. Whether you're looking to add to your collection or need a starter bottle, reusable water bottles are a simple way to help the environment, save money and stay hydrated. The main reason I carry one is to help eliminate the use of plastic — watch the documentary Bag It and you'll want to do something. Here's one thing. In a city full of fitness fiends, we have an extensive selection. Here's a guide to the city's coolest, all BPA-Free (BPA is a chemical compound used in manufacturing polycarbonate plastic and epoxy resins; it's an endocrine disruptor that mimics estrogen).