He proposed another parable to them. "The kingdom of heaven is like a mustard seed that a person took and sowed in a field. It is the smallest of all the seeds, yet when full-grown it is the largest of plants. It becomes a large bush, and the 'birds of the sky come and dwell in its branches.'"
He spoke to them another parable. "The kingdom of heaven is like yeast that a woman took and mixed with three measures of wheat flour until the whole batch was leavened."
All these things Jesus spoke to the crowds in parables. He spoke to them only in parables, to fulfill what had been said through the prophet: "I will open my mouth in parables, I will announce what has lain hidden from the foundation (of the world)."
Source: New American Bible, according to swiss-cath. reading order