Mixing hydrogen and oxygen equals water, but at room temperature and normal pressure this happens extremely slowly; an energy source, a catalyst, is needed to start the chemical reaction. However, hydrogen is extremely flammable and explodes even in the presence of a lighted match or a spark, so attempting this is not advised.
Demonstrations of what happens when a mixture of hydrogen and oxygen is ignited under controlled conditions are given by college professors for chemistry students, and the result is spectacular. The most famous example of an uncontrolled hydrogen/oxygen reaction is the 1937 Hindenburg disaster. This produced an enormous explosion and a lot of water. The danger of igniting hydrogen and oxygen is that hydrogen is flammable and oxygen supports combustion.
Clean water is an important concern throughout the world, and according to a 2006 United Nations report, up to 20 percent of the world's population is without it. In theory, it is possible to make water, but to do so on a global scale would require a large-scale process and would be extremely dangerous.
There are safer ways to produce water, and several methods have been invented. The most notable of these is AquaMagic, created by American inventors Jonathan Wright and David Richards. Their machine, used following Hurricane Katrina, sucks in air from the atmosphere surrounding it, passes the air through a refrigerated coil where it cools and then purifies and stores the collected water.