The other day I was in a worship and the pastor while preaching on some topic stated how electricity is invisible most of the times, but still how effectively it works. He said that when you 'on' a switch, the effect is seen somewhere, either a bulb lights up or a fan or a motor. The power behind the working of the object is rarely seen but it works nevertheless.
God spoke to me through this regarding my ministry of preaching and teaching. When someone hears when we preach the Word or someone reads what we write, God the Holy Spirit starts working. We can't see Him, but silently like electricity He touches those who have receptive minds or anyone for that matter and brings about the desired effect. Wonderful, isn't it?