Last night the Scouts were challenged: what do you do if you are out walking and your water bottle runs dry? They learnt how the empty plastic bottle could be fashioned into a DIY water filter, using stones, sand, charcoal scavenged from an old fire and a scrap of cloth.
Much to their amazement, when dirty brown water was poured in at the top, clear water came out the bottom. It would still need boiled, but it was the first step.