Either,
1. The stone pierced through his helmet; which such stones being slung would not seldom do; as even Diodorus Siculus relates. Or,
2. The stone might get in through one of those holes which are left in helmets, that he that wears it may see his way, and how to direct his blows. Or rather,
3. The proud giant had lift up that part of his helmet which covered his forehead; and that in contempt of David and his weapons, and by the singular direction of God's providence.