Today's Mass readings have Jesus talking about the "leaven" of the Pharisees and Herod. Back then there were no packets of yeast to buy at the store (good thing, too, because come to think of it, there weren't really any "stores"), so a bit of yesterday's bread dough (again, no stores for bread) was saved and inserted into the next day's dough to cause it to rise. Because this leaven was actually a piece of old dough that had "soured" (The Hebrew word for leaven, chametz, literally means "sour") and begun to decompose in order to release the gasses that cause the loaf to rise, it became synonymous with the whole idea of corruption. This is what the Lord meant by telling his 12 to "watch out" and "guard against" the leaven of the Pharisees and Herod.
This Jewish understanding of leaven is both "ancient and new." It is "Ancient", because Berachot 17a of the Talmud says, "leaven represents the evil impulse of the heart." "New", because even today, part of the traditional Jewish Passover preparations is to get rid of anything leavened in the house (all products containing wheat, barley, rye, or oats), a throwback to the bygone tradition that, when Passover came, no piece of the dough would be saved to be the following day's leavening agent. They ended the chain of the last bit of 'leaven' and "started fresh" with the holy day.
"A little goes a long way." We've heard it before. In the case of leaven or yeast, a little bit can affect the whole loaf (remember the classic "I Love Lucy" episode in which Lucy and Ethel bake bread?). Two little aspirins can make your whole body feel better, for another example. One little tick bite can infect your whole blood supply. The list goes on and on.
The first reading in today's Mass told of God's displeasure with humanity. Just six chapters after He created everything and it was all "good", He's ready to throw in the towel. A little bit of sin has entered the whole of humanity and corrupted the whole batch.
But this formula can also work to our advantage. Noah and his family were one small bit of humanity. But through them, the whole batch would be saved and God would not, in essence, "start from scratch." Noah, who "found favor with God", is seen as a foreshadowing of Jesus, God's Son "on whom [God the Father's] favor rests." Through our reception of the Eucharist, one small piece of bread, our whole body and soul is changed, and one little formula of absolution from a Priest can absolve a multitude of sins. Truly (as well as supernaturally), a little can go a long way!
Where can I affect a change, even if I'm "small and insignificant" in the world's eyes? At work? School? In my family?