This allusion comes from Volume 2 of Anecdotes of Yan State in Strategies of the Warring States .
故事出自《战国策》卷三十《燕策二》，大意是说：
A clam swam to the bank and opened its shell in the sun.
一只蚌游到岸边张开蚌壳晒太阳,
A snipe flew over and saw it,
一只鹬鸟飞过，发现了它，
so it dived to the ground and pecked at it.
就去啄它的肉，
Before the snipe drew back its beak,
没等鹬鸟把嘴回来，
it was gripped tightly inside because the dam suddenly felt a sharp pain and dosed its shell quickly.
蚌合上壳夹住了鹬鸟的嘴。
The snipe said angrily, "Listen, you clam, if it's not going to rain today, neither will it tomorrow. Then you'll die from thirst. "
鹬鸟说:“今天不下雨，明天不下雨，就会有一个死蚌。”
The clam was angry too. “You listen, I won't let you off today, and I won't let you off tomorrow, either. Then you’ll die of hunger.”
蚌也对鹬鸟说:“今天拿不出嘴米，明天拿不出嘴来,就会有一只死鹬。”
The snipe and clam were locked in fight and quarreled, and neither of them liked to give in first.
两个各不相让，
Just at that moment, an old fisherman came. He picked them up and took them home for dinner.
结果被渔人双双擒获。
The story tells us that the third party will have an advantage fall into his lap when the other two parties are locked in combat for their own personal interests.
后世便用“鹬蚌相争”一词来比喻双方因不必要的矛盾争执不下、互不相让，结果两败俱伤.反而让第三者占了便宜。
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