A:Once you break camp, a gravity water filter is the fastest way to purify water. Simply hang the dirty water bag high off the ground in a tree, and let the filtered water flow down to the other bag. For wilderness trips in the North America, any of these gravity filters will take out the nasty stuff—protozoa and bacteria. But to kill viruses when traveling internationally, you'll need to buy a different product that works as a purifier. Here are our three favorite filters for camping in North America.