New Defender's Study Bible Notes
17:40 five smooth stones. David only used one stone to slay Goliath (I Samuel 17:49). Possibly he knew that Goliath had four brothers who were also giants (II Samuel 21:22). These also were eventually slain, not by David, but by his men.
17:40 his sling. Slings were common and effective weapons at the time. Note Judges 20:16.