Pai Chang asked Yun Yen, "With your throat, mouth, and lips shut, how will you speak?"
Yun Yen asked, "Master, do you have any way to speak or not?"
Pai Chang replied, "I have lost my descendants."
In Buddhism, there is no subject separate from object, there is no self separate from other. How can there be speaking, then, which presupposes a speaker and a separate listener? How can there be teaching, which requires a teacher and a student? Pai Chang's question isn't literally how can you speak without your mouth, throat, and lips, but how can we use words without falling into the duality of self and other?
Pai Chang posed the question to three of his students, and all three responded with a statement back to him ("Please, Master, you speak instead," "Master, you should shut up," and "Master, do you have any way to speak or not?"). All three fell into the trap of relativism. None could pass Pai Chang's test. His answer to Yun Yen, "I have lost my descendants," acknowledges that none had answered his question.
If I had been there then, I'm sure I would have done no better than the three monks. But I think a good answer might have been, "With our ears shut, how will we hear?," or "There's no need to speak since we're all deaf." The awakened to not "hear" "others."
Kuei Shan, more commonly known as Guishan, and Wu Feng both eventually went on to become great Masters under the guidance of Pai Chang, also known as Hyakujo. Yun Yen, known as Ungan in Japanese, did not awaken to the Truth under Pai Chang, but did later when training under Yaoshan.
Yun Yen told Yaoshan, "Pai Chang once entered the hall to address the monks. Everyone stood. He then used his staff to drive everyone out. Then he yelled at the monks, and when they looked back at him, he said, "What is it?"
Yaoshan said, "You should have told me this before. Thanks to you today, I've finally seen elder brother Pai."
Upon hearing these words, Yun Yen awakened to the Truth.