We still read in the newspapers of people dying of thirst in the desert, but many of them could help themselves in this emergency. An experiment on a small scale in a sandbox will show you how to do it. Dig a fairly deep hole and place a beaker in the middle. Spread a suitably sized piece of transparent plastic foil over the edge of the hole and lay a small stone in its centre so that it dips down to the beaker in the shape of a funnel. The edges are fixed firmly into the sand. Soon, especially in sunshine, small drops of water form on the underside of the foil. They become larger and larger and finally flow into the beaker. The effect of the sun is to heat the ground strongly under the foil. The moisture held in the sand evaporates until the enclosed air is so saturated that small drops of water are deposited on the cooler foil. Even desert sand contains some moisture. If you also place cut up cactus plants into the hole, you will obtain enough water to survive.