Ex: - "chán như con gián": it means "bored". If you translate it word by word or use googletraslate, you have "bored as cockroaches". Actually, a cockroach - "con gián" and boring - "chán" are not related in meaning. However, in this case, Vietnamese is used to repeat rhyme "an" betweet "chán" and "gián" to make intonation. This make listener easily to remember a sentence. The repeat rhyme is especially used in poems. - "Buồn như con chuồn chuồn" is same meaning to "chán như con gián". Rhyme "uồn" is repeated. "Buồn" - "sad" is not related to "chuồn chuồn" - "daragonfly" in meaning. - The continuous expanding and development of Vietnamese slang is similar to English slang nowadays.