Yeast is a minute plant, and like other plants must have the right conditions of heat, moisture, and nourishment in order to live or to flourish. It will be killed if scalded, or if frozen, as any other plant would be; therefore, as we depend upon the growth of this little plant for raising our bread, we must give its requirements as much care as we do our geraniums or our roses. The yeast plant takes its nourishment from sugar. This is found in flour. It converts this sugar into carbonic acid gas and alcohol, and the pressure of this gas causes the mixture in which it is generated to become inflated, or to "rise".
In mixing bread, we put the yeast into warm (not hot) water; this we mix with flour, thus supplying the moisture and nourishment required. We put this mixture in a warm place to force the growth of the plant. When the dough has become sufficiently inflated we put it into the oven and raise the heat to a degree which kills the plant and fixes the air cells, and our bread is done.