Once upon a time, the State of Chu planned a night attack on the State of Song, but the Yong River posed a natural barrier.
To overcome this, Chu sent men to secretly measure the river's depth and set markers for wading across at night.
However, when night fell, the river's water level suddenly rose. Ignoring the change, the Chu army stubbornly attempted to cross at the marked spots from daytime. This led to disaster: over a thousand soldiers drowned. Desperate cries and chaos erupted on the river, and the army collapsed and was routed instantly.