The growing gap between rich and poor, not only in income, but also in the provision of basic services comes home to us when we realise that the combined income of the richest 500 people in the world exceeds that of the poorest 416 million. A recent UNDP report reveals that 2.4 billion people worldwide do not have access to safe sanitation, and that water-borne diseases such as diarrhoea kill far more people than HIV/AIDS and malaria combined and five times as many children as HIV/AIDS. In many areas very poor people pay more per litre for private water supplies than in New York or London. In addition to the loss of life and health costs, the time spent collecting water has huge economic effects. The report calculates that the cost to Africa is equivalent to about 5% of the continent’s economic growth. In this devastating context a project begun in Tanzania could be a small step forward towards a safe and inexpensive method of sterilising water. The local water supply to Ndolela village is not clean enough to drink and when the pipes run dry, villagers generally fetch water from a dirty spring. Now about 40 houses in Ndolela have begun to use a simple method of solar purification to ensure that a combination of ultra-violet rays and heat destroys the bacteria causing common water- borne diseases like cholera, typhoid, dysentery and diarrhoea. Villagers fill plastic bottles with contaminated water and place them on black-painted roofs. After eight hours in the sun, the water is safe to drink.