The World Cannot Exist With Only Goodness

The milkman approached the Rebbe and told him that he had done what was right but it had ruined his milk! The Rebbe told him, "You should know, my son, that since the sin of the eitz hadaas, good and bad are mixed together, and the world cannot live with only goodness. What you should do is to put one drop of water into the milk fulfill the obligation to have a bit of bad mixed with the good."