David vs. Goliath

Supposedly these weren't just pebbles, they were more like the size of a gapefruit

Maybe it was a small pebble straight through the eye?

David's hand was guided by god. The bible states the stone sunk into Goliath's forehead, that means it smashed through bone. It was to show that faith in god will make the simplest things more powerful then the greatest that man has to offer.