A pre-ferment is a preparation made from a portion of the dough’s ingredients that is allowed to ferment in advance and is then added to the final dough. The water used to make a pre-ferment is generally cool, that is, about 70°F (20°C). However, no two pre-ferments are alike. Some are loose and runny, some stiff, some salty, some unsalted, some made with commercial yeast, others from natural wild yeasts, and some are simply a small percentage of a mixed bread dough. Because pre-ferments vary dramatically, they can present quite a challenge for the baker, who otherwise requires such precision in all things.