cook sand for rice—a useless attempt ❍ 当然，试一试并没有坏处，但你的尝试显然是～。Of course,there is no harm in trying,but yours is obviously a useless attempt just like cooking sand for rice.
炊沙作饭chuī shā zuò fàn
煮沙子作饭。比喻做无用功，白费工夫。impossible, incapable of occurring or being done, a useless attempt