invisible ink       
noun [ U ] uk/ɪnˌvɪz.ɪ.bəl ˈɪŋk/ us/ɪnˌvɪz.ɪ.bəl ˈɪŋk/
 隐形墨水
ink that cannot be seen until it is treated with chemicals or heat
  It's a secret message written in invisible ink. 这是一条用隐形墨水写的秘密消息。 
 
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