A minister had several good illustrations that he used effectively in a sermon on "living water." At one point he said, "God's living water is like the water flowing in the Mississippi River. It's free." 

After the sermon a skeptic came up to him and observed, "If the water in the Mississippi river is free, why do you pass the plate to take up a collection during the church service." 

Unfazed the pastor replied, "Well, while the water is free, you do have to pay to 
get it piped directly to your door."