"How can this be?"
One way the question can be answered is in the activity of God. He is involved in His creation. Many of the forefathers of the USA were Deists. They believed that God was like a watchmaker who, having made the watch, winds it up and lets it work on its own. They felt that God watched from a distance as His creation worked on its own. Nothing could be further from the truth! God made the watch, keeps it ticking, and fixes it when it breaks.
David knew about the activity of God. God had helped him many times. Now, as recorded in our scripture reading, David faced the fight of his life. The whole nation of Israel depended on the defeat of Goliath. David made it clear that the LORD would give him victory. He got into the battle by convincing King Saul that "The LORD who rescued me from the claws of the lion and the bear will rescue me from this Philistine. He said to Goliath, "You come to me with sword, spear, and javelin, but I come to you in the name of the LORD of Heaven's Armies...Today the LORD will conquer you. And He did!
We face many giants in our lives. There are dangers, difficulties and doubts. But the same God who delivered David and rescued the whole world through Christ will give us the victory. This is a call for faith. We should take God at His word by taking His Word into our lives. And then, with the promise of God's activity in our lives go out and slay our giants(even dwarfs that seem to us like giants!). We know He will do it because He did it that first Christmas. Jesus means "Savior". And Savior means "Deliverer". That's what He came to do. And He will!