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.