立竿见影

Origin and story

The Cantong qi, traditionally associated with Wei Boyang, pairs two immediate responses: a standing pole produces a shadow, and a shout in a valley produces an echo. The passage uses these familiar images while describing the efficacy of an alchemical process. They function as comparisons, not a separate story about a person inventing a measuring device.

The idiom carried the pole-and-shadow image into general use. It describes an action whose result becomes apparent at once, just as a shadow is visible when a pole is placed in sunlight. The underlying alchemical claims belong to the text's historical setting; the everyday expression does not depend on accepting them.