Mark 4:26-29 (NKJV)
And He said, "The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come."
A farmer knows how to grow and harvest a crop. But does he really know what is happening? The above verse says that our crops of fruit and vegetables come from the earth. Basically a seed has the ability to build a wonderfully sweet, succulent cantaloupe out of, well...dirt, air and water basically. And it attaches itself to a solar energy source as well. It was not until recent times that man has begun to understand the workings of DNA and the marvels of life. However understanding how a thing works is not the same as understanding how it came to be.
Why does the seed produce a live plant? Because the seed contains life itself. In the same way, the word of God which is sown in men's hearts produces life. All that is required is fertile ground which is prepared to receive the seed.