1 Corinthians 9:11

If we have sown unto you spiritual things, is it a great thing if we shall reap your carnal things? (KJV)

Then: The first of four more rhetorical questions links material and spiritual concerns. There is no difference in the work sown for spiritual harvests as that for physical harvests. The apostles still need to eat and live by reaping physical support from the harvest. Fee (p. 409) calls it a reciprocal interpretation. The Corinthians received spiritual benefits to live spiritually; the apostles deserve physical benefits to live physically.

Now: Both the principle and the teaching technique of asking rhetorical questions are still useful today.

Extra source: The First Epistle to the Corinthians by Gordon D. Fee (Eerdman's 1987) online at http://books.google.com/books?id=XlBp10nUTXAC&dq=Gordon+Fee+Bible+commentator&printsec=frontcover&source=in&hl=en&ei=SwLNSu3nA4v6Mf2XlDo&sa=X&oi=book_result&ct=result&resnum=12#v=onepage&q=&f=false