TOKYO-A Japanese student of ninja history who handed in a blank paper was given top marks - after her professor realised the essay was written in invisible ink. Eimi Haga followed the ninja technique of “aburidashi”, spending hours soaking and crushing soybeans to make the ink.

The words appeared when her professor heated the paper over his gas stove. “It is something I learned through a book when I was little,” Haga told the BBC. “I just hoped that no-one would come up with the same idea.”

Haga has been interested in ninjas - covert agents and assassins in medieval Japan - since watching an animated TV show as a child. After enrolling at Mie University in Japan, the first-year student took a class in ninja history, and was asked to write about a visit to the Ninja Museum of Igaryu. “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,” she said. “I gave a thought for a while, and hit upon the idea of aburidashi.”

Haga, 19, soaked soybeans overnight, then crushed them before squeezing them in a cloth. She then mixed the soybean extract with water - spending two hours to get the concentration right - before writing her essay with a fine brush on “washi” (thin Japanese paper).