See these parables explained in the notes at Matthew 13:31-32.
Related Commentaries of Luke 13:18
Luke 13:18-21
18 Then said he,Unto what is the kingdom of God like? and whereunto shall I resemble it?
19 It is like a grain of mustard seed, which a man took, and cast into his garden; and it grew, and waxed a great tree; and the fowls of the air lodged in the branches of it.
20 And again he said,Whereunto shall I liken the kingdom of God?
21 It is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened.