The oldest and commonest is by mixing in with the flour and water a small amount of the frothy mass made by a germ, or microbe, known as _yeast_ or the _yeast plant_.

Then the dough is set away in a warm place "to rise," which means that the busy little yeast cells, eagerly attacking the rich supply of starchy food spread before them, and encouraged by the heat and moisture, multiply by millions and billions, and in the process of growing and multiplying, give off, like all other living cells, the gas, carbon dioxid.

This bubbles and spreads all through the mass, the dough begins to rise, and finally swells right above the pan or crock in which it was set.

If it is allowed to stand and rise too long, it becomes sour, because the yeast plant is forming, at the same time, three other substances--alcohol, lactic acid (which gives an acid taste to the bread), and vinegar.

Usually they form in such trifling amounts as to be quite unnoticeable.