And she asked him: what it would be like the Kingdom?

And he said: It's just like a seed, a single grain of mustard seed.                                          Which a woman took and sowed in her garden. And it grew and it grew.                      And the birds of the air come and nested in its branches. 

The Mustard Seed parable highlights the idea that something seemingly insignificant can carry immense significance. Although not the smallest seed known, the mustard seed's humble beginnings blossom into astonishing greatness, illustrating the theme that many great things start small.