A Japanese student has been awarded top marks for handing in a blank piece of paper after her professor realised she’d written her entire essay in invisible ink.
Eimi Haga, a first-year student of ninja history at Mie University, applied the ninja technique of “aburidashi” to an assignment.
It involves crushing soaked soya beans to create an invisible ink that can only be viewed when it held near heat.
The 19-year-old had read about the technique when she was younger and decided to use it to make her essay - written about a visit to the Ninja Museum of Igaryu - stand out.
"When the professor said in class that he would give a high mark for creativity, I decided that I would make my essay stand out from others," Haga told the BBC.
"I gave a thought for a while, and hit upon the idea of aburidashi."
"It is something I learned through a book when I was little," she continued. "I just hoped that no-one would come up with the same idea."
To ensure her professor didn’t miss the secret text, Haga included a note instructing him to “heat the paper”.
He duly did and when the text appeared, immediately gave Haga’s essay full marks - despite admitting that he still hasn’t read it to the end.
"I had seen such reports written in code, but never seen one done in aburidashi," Yuji Yamada said.
"I didn't hesitate to give the report full marks - even though I didn't read it to the very end because I thought I should leave some part of the paper unheated, in case the media would somehow find this and take a picture."
Although ninjas are a popular -if misrepresented - stereotypes in Western depictions of Japan, the real ninjas were espionage agents who practice ninjutsu, a type of guerilla warfare.
The tradition dates back to the feudal period but the arts practiced by ninjas have all but died out in modern Japan.
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