Solar disinfection of water is beautifully simple. All you need is tropical sunshine, a plastic bottle full of water and some patience. The GATE / Small-Scale Project Fund is pioneering solar water disinfection in Sri Lanka. Transparent plastic bottles are filled with water and placed in the sun for six hours. Laying the bottles on the kind of corrugated tin or iron used for roofs works particularly well. That is enough time to disinfect contaminated water through heat in order to make it safe for drinking. This technique is particularly effective for removing pathogens that cause diarrhoea. If the temperature is below 42oC or it is cloudy, two days does the trick. If the water is too cloudy to see the label on the far side of a clear bottle, it needs to be filtered first. The pilot project has already cut water-related disease rates in nursery schools.