A first-year Japanese university student scored an A for her essay on the history of ninja – by writing it in invisible ink.
At first sight it looked as though Eimi Haga had turned in a sheaf of blank papers for her assignment at Mie University, the Asahi Shimbun reported.
But the 19-year-old had actually adopted a ninja communication technique in which characters are written with crushed soybean extract – turning invisible when dried and only seen again when heated, a technique known as aburidashi, dating back to the Edo Period (1603-1867.)
Haga included a brief note asking for the essay to be held over a flame.
Her professor, Yuji Yamada, told the BBC he was “surprised” when he saw the essay.
“I had seen such reports written in code, but never seen one done in aburidashi,” he said.
“To tell the truth, I had a little doubt that the words would come out clearly. But when I actually heated the paper over the gas stove in my house, the words appeared very clearly and I thought ‘Well done!’”