Why did David take five smooth stones? Basically because there wasn't just one giant. There were actually five of them. David was fully anticipating and ready, if need be, to slay all five giants. Judges 3:1-3: "Now these are the nations which the Lord left, to prove Israel by them, even as many of Israel as had not known all the wars of Canaan; Only that the generations of the children of Israel might know, to teach them war, at the least such as before knew nothing thereof: Namely, five lords of the Philistines..."