This is a story about how the most amazing ideas do not always work how you would like in practise. In theory it is so simple. You put disease-ridden water into a two litre plastic bottle, screw on the lid and leave it in the sun. After six hours on a cloudless day, almost all the bacteria and bugs that cause diseases like cholera and diarrhoea are killed or inactivated by the UV light and gentle warming. Professor Kevin McGuigan has proven this in the lab, but for the last 20 years he has been trying to get it working in rural African communities. It has not been anywhere near as easy as you might think. (Photo: Godfrey putting his water bottle out to disinfect in the sunshine)