Idiom Magic﹕How high is a Chinaman? 誰知道?

【明報專訊】This odd expression is given in reply to a question that is impossible to answer. It's like asking someone how dark night is or how deep space is. It means there is no way to judge or decide something, and it has no other meaning or significance (含意). "When Peter asked if there was life on Mars I shrugged (聳肩) and replied how high is a Chinaman?"