Before John Piper preaches he often prays that God would give him prophetic insights that would drive the truth home for his listeners. On one occasion this happened in a dramatic way.
John was preaching on small groups and reminding the congregation of the fact that you can set up a small group anywhere. He encouraged people to think about trying investigative Bible studies with their non-Christian friends. Then, off script, he gave an illustration. 'You might even work on the 34th floor of the IDS tower. Maybe you should call your people together and have a small group meeting.'
After the service a woman approached him and asked, 'why did you say that?'
It turns out that she worked on the34th story of the IDS building and had been praying about starting a small group.