The gas bubbles up through the dough, and this is what makes the bread light.

This is called _fermentation_ (fer-men-ta'-tion).

The little alcohol which is formed in the bread does no harm, because it is all driven off by the heat when the bread is baked.
[Illustration: FERMENTATION.] ~10.~ Any moist substance or liquid which contains sugar will ferment if yeast is added to it, or if it is kept in a warm place.

You know that canned fruit sometimes spoils.

This is because it ferments.