鹬蚌相争 (yù bàng xiāng zhēng)
One day a clam opened its shell to sunbathe on a beach. Suddenly a snipe stuck its beak in the clam. The latter closed its shell immediately, and trapped the snipe’s beak. The clam refused to open its shell, and the snipe refused to remove its beak. Neither of them would concede defeat. Finally, a fisherman came along and caught both of them. This idiom means that if two sides contend, it is a third party that benefits.