Prayer #3 - Why do Calvinists Pray?

In this second sermon on prayer we examine the question as to why people who believe in God's sovereignty pray? The harder question is why do people who don't believe in it pray? We have every reason to pray, as we see prayer as one of the means God uses to establish His sovereign will, but for those who don't believe God interferes with man's free will- what can God do in answer to their prayer?