One of the problems with water bottles is their bulkiness, especially when empty. The Vapur water bottle collapses and rolls up into a tiny package about the size of a change purse. It’s BPA free and dishwasher-safe. It weighs about 1 oz, you can fill it with water and freeze it. There’s a carabiner on it so you can attach it to stuff to carry it. Capacities range from 0.4 liter to 1 liter-sized bottles. The manufacturer sells replacement caps and carabiners, and a new kind of bottle with whose flip-top cap has a built-in carabiner (I do not have personal experience with, but looks like an improvement on the older style). There’s even a variety with a built-in filter for outdoor use. I’ve had a couple of these for about a year and it’s become my gym bottle of choice.
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