意思 yi4si idea
川流不息 chuan1liu2bu4xi1 the river flows endlessly
思 yi4 & 息 xi4 illustrate a common problem of Chinese characters: similarity.
息 xi4 means: breath news interest cease stop rest. Its the nose above the heart so the idea is the person stopped breathing, to rest; alternatively that they are getting a “whiff of the news”.
思 si1 in contrast is the brainbox on top of the heart & means “idea”.