cook sand for rice—a useless attempt ? 當(dāng)然，試一試并沒(méi)有壞處，但你的嘗試顯然是～。Of course,there is no harm in trying,but yours is obviously a useless attempt just like cooking sand for rice.
炊沙作飯chuī shā zuò fàn
煮沙子作飯。比喻做無(wú)用功，白費(fèi)工夫。impossible, incapable of occurring or being done, a useless attempt