This article is from the rec.food.drink.beer FAQ, by John Lock with numerous contributions by others.
Lambics are a type of ale brewed in parts of Belgium by exposing hot
wort (unfermented beer) to the outside air. Indigenous, wild yeasts
and other microorganisms settle on the exposed surface of the wort as
it cools and begin spontaneous fermentation. They are often sweetened
with fruit flavorings and generally prized the world over.
 
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