On average, the human body comprises 50-60% water. During and after sport or on hot days, we sweat. It's therefore important to compensate for this loss in fluid by drinking extra water. Normally, for shorter expeditions, a decent water bottle or two will suffice. Look for a model that uses BPA-free plastic and has the right volume and cap for you. If you're out for longer, you may run out of water. This isn't the end of the world, because you can drink natural freshwater. However, you do have to be careful: use a portable water filter or purification tablets to protect yourself from contaminated water and bacteria. In this category, you'll find water purification aids as well as reservoirs and a variety of water bottles.