If you have a clear plastic bottle, some clear water, and a sunny day, you can use the sun's light to make your water much safer to drink. Largely advocated for developing countries, solar water disinfection (aka SOLDIS) is gaining traction in the survival-skills crowd; and it's a great fit for equatorial countries with abundant strong sunlight but few other resources. The most common solar disinfection technique is to expose clear plastic bottles full of questionable water to the sun for a minimum of one day. The sun's abundant UV light kills or damages almost all biological hazards in the water. There are some problems, though. You need sunny weather (or two days of overcast sky) to reach the maximum effectiveness. You cannot use it in rainy weather. It offers no residual disinfection. It may be less effective against bacterial spores and cyst stages of some parasites. Both the water and the bottle need to be very clear. And finally, it only works with bottles that are 2 liters or smaller. While solar disinfection isn't 100 percent effective, it's still a lot better than taking your chances by drinking raw water.