In ancient times, shepherds needed a system to keep track of their flocks. On some days, they would go out to pasture with a flock of 30; on others, a flock of 10. Memory was an unreliable way of keeping tabs on the number of the flock. As a result, the shepherd would carry a sling over his shoulder, and in it he would keep the number of pebbles that corresponded to the number in his flock. That way he could at all times have an accurate daily count.
Fast forward a few years. The Book of Kings tells us that David became the Champion of Israel and went out into the field of battle with only his sling and five smooth stones. He was to mee the giant Philistine, Goliath.
We all know the story. Goliath insulted and David slung (silinged, slang?) a stone, hit Goliath in the forehead, and rush to him, taking his sword and cutting off his head.
But, why did David take 5 stones? Answer: Goliath had 4 brothers.
So let it be written, so let it be done.