The pipe is hollow and can transmit sound. The sound of a stone falling into water can be completely relayed through the pipe. By carefully listening, one can definitely discern whether the surface below is ground or water. Cao Xi has ancestral skills; he can not only hear the sound of the water, but also gauge the approximate depth.
Luo Zheng also understands the principle of sound transmission through the pipe and excitedly asks, "Great! What do we do next?"
