Christ said that the kingdom of heaven could be likened unto leaven (or yeast), which a woman took and hid in three measures of flour, till the whole was leavened.
Now, this leaven, or yeast, is composed of tiny little plants, each one so small that it cannot possibly be seen by the sharpest eye except through a very powerful microscope. So small are they that it would require three thousand of them, placed close together, side by side, to make up the length of one inch. Like all other plants they require food, and they find this in the dough they are placed in. You know that all things are made up of atoms of chemical substances so wonderfully blended together that only the chemist can separate them, and when he has separated them they appear very different. Well, in flour there are certain things so blended, and the yeast-plant takes one kind of substance as food, and in doing so sets free another substance called carbonic acid gas. This gas bubbles up and makes the heavy dough spongy and light. If it were not for these tiny bubbles of gas your bread would be as heavy and close as suet pudding. This is the reason why yeast is put into dough for making bread or cake. One of the most remarkable things about this yeast is, that when it gets into any substance that contains its food, it at once begins to give off buds, which, in a few moments, become full-sized and break away. So rapid is this increase, that if a single yeast-plant were to be put into a great mass of dough it would very quickly leaven the whole mass.
And so it is with the love of God. When once it gets into our hearts it will keep on growing until all our life is filled with it, and we try in all things to please Him.