A man cleared stones from his own field and threw them onto the public road. A pious man passing by saw this and rebuked him: "Fool, why do you throw stones from a field that is not yours into a field that is yours?"
The stone-thrower laughed at the absurdity of the statement. The field was clearly his — he owned it, he farmed it. The road was clearly not his — it belonged to everyone and no one. How could the pious man have it backward?
Years passed. The man's fortunes turned. Through debt or disaster, he lost his field. Now he had nothing — no land, no home, no livelihood. He walked the public road, the same road where he had once thrown his stones.
And he stumbled on them. His own stones tripped him, bruised him, blocked his path on the very road he now depended upon. The Talmud (Bava Kamma 50a) records that he then understood what the pious man had meant.
The field was temporary — he could lose it at any time. The road was permanent — he would always need it. He had damaged what was truly his (the public commons he depended on) to improve what was never truly his (the private property that fortune could take away).
The sages used this parable to teach about charity and community. Those who enrich themselves at the public's expense are throwing stones onto their own road. When fortune turns — as it always does — they will stumble on the very obstacles they created. The community is the road every person walks. Damage it, and you damage yourself.