As he went along, he saw a man blind from birth. His disciples asked him, "Rabbi, who sinned, this man or his parents, that he was born blind?"
"Neither this man nor his parents sinned," said Jesus, "but this happened so that the work of God might be displayed in his life. As long as it is day, we must do the work of him who sent me. Night is coming, when no one can work. While I am in the world, I am the light of the world."
Having said this, he spit on the ground, made some mud with the saliva, and put it on the man's eyes. "Go," he told him, "wash in the Pool of Siloam" (this word means Sent). So the man went and washed, and came home seeing.
-So, This guy was blind, and the disciples thought that it was his parent's fault, or even his fault that he was born blind. It turns out, the man was only blind because God wanted to show His work in the man's life. Jesus then healed the man, so that He could see. Even though we may not be blind, God uses our weaknesses to grow us stronger, and to show His work though us.
We are to do His work as long as it is day (or as long as we live on this earth), Jesus goes on to say "While I am in the world, I am the light of the world."
Well....The Holy Spirit lives in us, so we are to show the Lord's light while we are in the world!