"Or how can you say to your brother, 'Let me take the speck out of your eye,' while the log is in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your neighbor's eye" (Matthew 7:4-5).
Jesus was, among other things, a skilled teacher. He was teaching orally, of course, since few if any of his followers could read. He knew that exaggeration and humor would help his students to remember more clearly. His teaching here is that no one is perfect, so no one can judge his 'brother' with impunity. We must first 'heal ourselves' of our own misconceptions before the can lovingly (without judgment) help others on their own spiritual paths. He makes the point vividly by creating the image of a speck of dust in one eye and an entire log in our own. I think it's still a human tendency to see minor flaws in others more readily than we notice our own imperfections.