There are two chimney sweeps, the two go down the chimney to clean, one emerges with his face all sooty, the other emerges with the clean face? who washes?
There are two chimney sweeps, the two go down the chimney to clean, one emerges with his face clean, the other comes out all dirty and sooty, who doesn't wash?
In the first scenario, the one who comes out clean will wash his face because he sees that his companion's face is all dirty, so he imagines that his face is dirty too, so he will wash!
In the second scenario, the one whose face is dirty will not wash, because he sees that his companion's face is clean, so, he imagines that his face is clean too, so, he will not wash.
How do we apply these scenarios in a spiritual and moral way? the one who washes his face, in spite of the fact that it is clean symbolizes the one who realizes that another person is tamei (unclean) but even though he knows he's clean, looking at his neighbor makes him think, there might be small areas in his life that are not right before HaShem, so, just to be sure, he meditates on his life, and finding perhaps some falts, does "Teshuvah" because, it is hard to come down a chimney and not pick up a few specks of soot.
The other scenario is about the chimney sweep with the dirty face, and sees the clean face of his companion, and does not wash, symbolizes the one who sees the face of a Tzadik, and, in pride, he says to himself, "I too am righteous" and thus deceives himself, and remains "tamei" so, be like the one who, in spite of having a clean face, still washes.