Mark 4:26-34 NIV
He also said, “This is what the kingdom of God is like. A man scatters seed on the ground. Night and day, whether he sleeps or gets up, the seed sprouts and grows, though he does not know how. All by itself the soil produces grain—first the stalk, then the head, then the full kernel in the head. As soon as the grain is ripe, he puts the sickle to it, because the harvest has come.”
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.
NIV: New International Version