A hydration system is an apparatus used in recreation and other sustained outdoor activities. It is intended to help its user carry liquid, to support the physical effort involved in the activity, without the need to use one's hands or take off the pack. Such systems for consumers were first sold to cyclists, and by the 1990s had also found a substantial market among hikers. Familiar commercial models can also be recognized occasionally worn by western military personnel in southwest Asia. In practice, such a system is almost always a commercially manufactured unit that features at least a flexible bladder of one or a few liters' (quarts') capacity with some means, usually a screwtop, to fill and then reliably seal it, a light hose to convey the beverage to the user's mouth, and a bite valve that starts and stops the flow through the hose with minimal effort. Also common are designs that include specific hands-free means to comfortably carry the hydration system. History The plastic bladders from box wine were repurposed as an inexpensive way to store water when hiking in Australia in the late 1960s but were prone to breakage and leakage. Hydration systems first appeared commercially in the late 1980s or early 1990s, at backpacking stores. They were adopted by US special operations troops in the early 1990s, and became standard issue for all US troops in the later 1990s. The concept of the hydration system appeared in Robert A. Heinlein's 1955 novel, Tunnel in the Sky, where the main character has "a belt canteen of flexible synthetic divided into half-liter pockets. The weight was taken by shoulder straps and a tube ran up the left suspender, ending in a nipple near his mouth, so that he might drink without taking it off." Hardware Bladder designs The overall geometry of the bladder is nearly universal: The largest dimension is the vertical one, taking advantage of the long vertical dimension of the human torso. The hose joins the bladder very near the bottom of the bladder, to maximize the amount of accessible liquid. The bladder's change in volume as it is gradually emptied is reflected in the decrease of its smallest dimension (thickness); this lets the vertical and side-to-side dimensions stay fixed, thereby limiting the shifting of it and any items next to it, and minimizes the flexing of the bladder walls. Packaging Typical commercial hydration systems are