Eimi Haga handed in what looked like a blank sheet of paper - but left her professor a crucial clue.
Secrets. Genius.
Ms 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.
Eimi Haga followed the ninja technique of "aburidashi".
How she did it.
Soaked soybeans overnight
Crushed them before squeezing them in a cloth.
Mixed the soybean extract with water - spending two hours to get the concentration right
Writing her essay with a fine brush on "washi" (thin Japanese paper).
Once her words had dried, they became invisible.
To ensure her professor didn't put the essay in the bin, she left a note in normal ink saying "heat the paper".
Professor Yuji Yamada
"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!'
