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<nomobile>[[File:Poolish.jpg|thumb|left|upright=.7|Poolish - above, freshly mixed; below, risen]]</nomobile>
Centuries later, once microscopes were around, the bubbles that cause the dough to rise were shown to be produced my microörganisms. Oversimplifying, there are two kinds of them. One kind is lactic-acid bacteria, which eat the sugars present in the flour, excreting carbon dioxide and lactic acid. Bacteria like these are also responsible for milk going sour (hence their name: "lactic" means "milk", "acid" means "sour") and for cabbage turning into sauerkraut. In bread, the carbon dioxide produces the bubbles which make the bread fluffy, while the lactic acid gives the bread its specific tart flavour. The And yes, the dough which is left for bacteria to make it sour is called, you guessed it, sourdough.
The other kind is a bunch of single-celled fungi found in the froth from beer brew and called yeast. The yeast eats sugar, excreting carbon dioxide and ethyl alcohol. Yeast like this is also responsible for producing wine, mead and vodka. In bread, the carbon dioxide makes the bread fluffy, while the alcohol mostly evaporates during baking (which means you can't get drunk on bread; tough luck). Bakers, as we've already seen, used to obtain their yeast from breweries and add it to the starter for their dough (not sour at all).