Peter comes to the house where everyone is praying for him. A girl named Rhoda answers the door, sees it's Peter, slams it and goes to tell the others. They were praying for him, but they didn't believe it would really make any difference. This made me remember a story about some good church going people who gathered for Wednesday night prayer meeting. They were in the midst of a drought, so they were praying for rain that night.
Prayer is powerful.
Prayer does change things and often changes us. So be careful when you use it. Be expectant, too. Listen for that knock on the door and keep those umbrellas handy. Lord, we believe, but help thou our unbelief.