Again he said, “What shall we say the kingdom of God is like, or what parable shall we use to describe it? It is like a mustard seed, which is the smallest of all seeds on earth. Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds can perch in its shade.”
With many similar parables Jesus spoke the word to them, as much as they could understand. He did not say anything to them without using a parable. But when he was alone with his own disciples, he explained everything.