In The Gambia, sour milk (locally known as 'sowe') is actually a delicacy, usually consumed with porridge. It is made by leaving out normal cowmilk until it ferments. The top part - locally called 'feneh' - is then skimmed off.
Both Sowe and Feneh can be bougth at the market, or from women who go around from house to house each morning selling it (in a similar way to the milkman in other countries). Feneh is generally more expensive, and tastes better with porridge.