a mustard seed
It might look small now, but it will become something great, so great, it will be all there is one day (Mt 13:30).
Ask the students to each summarize what they think the mustard seed symbolizes. Through brain storming, help students see that even if we are not sure what the mustard seed represents, we can still understand that the Kingdom is going to start off small and become something huge. Maybe this means that the Kingdom will start with only a few who are loyal and true to the King, but eventually the Kingdom will grow and many will bow to Jesus.