Rehydration salts are the dry mixture of minerals and glucose that, dissolved in clean water, makes an oral rehydration solution. The standard World Health Organization formula contains sodium chloride, potassium chloride, trisodium citrate, and glucose in precise proportions: the glucose enables sodium, and with it water, to be absorbed efficiently through the intestinal wall even during diarrhoea. The salts come in sachets for mixing with a measured volume of safe water, and the resulting drink is the frontline treatment for dehydration from diarrhoeal disease, which still kills hundreds of thousands of children yearly. Cheap, stable, and needing no refrigeration, rehydration salts are counted among the most cost-effective life-saving interventions in global health.