“Bodo cracked nuts with a stone”
One day he found a smooth stone that had a little pit on either side.
He put his thumb and finger into the pits and cracked a hard nut with the stone.
This was just what he needed.
It neither slipped nor jarred his hand.
Some of the other Tree-dwellers tried it.
They wanted one like it, so they began to hunt for pitted stones.
They could not always find such stones, but they never thought of making the pits.
People lived many years before they learned to do that.
Tree-dwellers simply used things that they found on the spot.