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A pen that conceals what is written is a big hit with Chinese teenagers.

The pen, also called "invisible pen", uses special ink which left no marks on papers, while can be read with a mini purple lamp on the other end of the pen.

The pen is popular with students in Wuhan who are using it in exams or writing love letters.

"The pen should totally be forbidden. It is bad to the mental health of the students," said the principal of a local school.

But students disagree. A girl student named Wang said: "The pen has irreplaceable advantages. My mom always loves to peek at my diaries, and with this pen, she has no idea," reports Chutian Golden Newspaper.