There is a story in 2 Kings which goes some way to answering this:
“38 Elisha returned to Gilgal and there was a famine in that region. While the company of the prophets was meeting with him, he said to his servant, “Put on the large pot and cook some stew for these prophets.”
39 One of them went out into the fields to gather herbs and found a wild vine and picked as many of its gourds as his garment could hold. When he returned, he cut them up into the pot of stew, though no one knew what they were. 40 The stew was poured out for the men, but as they began to eat it, they cried out, “Man of God, there is death in the pot!” And they could not eat it.
41 Elisha said, “Get some flour.” He put it into the pot and said, “Serve it to the people to eat.” And there was nothing harmful in the pot.”
The question that struck me was: why flour?
I am assuming flour is not usually used as a cure-all against poison from unknown herbs. I am also assuming there is nothing special about this particular flour: it was not magical or super-powered in any way! This is just like the staff of Moses: it was not an ancient magical heirloom. It was a stick Moses happened to own, but God used it to turn the Nile into blood.
I think the point is that God chose this flour and from that point on nothing else would do. If Elisha had just thought up the flour, it would not have worked. If he had used salt, as he had done earlier to purify water, it would not have worked. The reason that Elisha knew what to do was that he was paying attention to what God was saying to him at the time. There is no formula to predict what God will use- we just have to listen.
Another lesson that comes out of this is that God can use things and people in unexpected ways, and not because of anything special about us or the things around us. As a brother pointed out, this is true even of how Abram was chosen. He was just a member of an ordinary people who worshiped idols. But God chose him, and he listened. So, who does God choose? That’s up to Him! If we have chosen to believe in Jesus and his promises then He first chose us, through no merit of our own. And what will God use? That too is up to Him. Our job is to listen and see what He is doing next so we can be part of it!