Guǎnggào shuō de tiānhuāluànzhuì, mǎi huí lái què dà shī suǒ wàng.
The advertisement was full of extravagant claims, but after buying it I was deeply disappointed.
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history
The idiom originates from Buddhist scripture: when the monk Yunguang gave a magnificent sermon, flowers were said to rain down from heaven. 天花乱坠 now ironically describes speech that is dazzling but empty.