A professor in Japan had to concede that a student in ninja history knew her stuff when she turned in an essay in invisible ink that she'd made herself. The essay came with a note written in regular ink saying to heat before reading. Once the professor held the paper over a hot stove, the words appeared, the Guardian reports. "She replicated what is written in records of ninja art," the Mie University professor said. "She strived to prove what was written actually works." For that, Eimi Haga, 19, received an A. "I was impressed," her professor said, adding that he'd received assignments written in code, but never in invisible ink.
After her professor said he'd give extra credit for creativity, Haga started thinking. She decided to try the ninja technique of aburidashi—"something I learned through a book when I was little." The process began with soaking soybeans overnight, per the BBC, then crushing them. She squeezed them in a cloth and mixed the extract with water, a stage that took two hours of experimenting to achieve the right concentration. Then Haga wrote the essay with a fine brush on thin paper. The essay was all about the ink: The professor conceded he didn't read the essay to the end, in case he wanted to demonstrate how it worked later. Haga, a member of the school's ninja club, was confident that her ink mastery would bring a good grade, "though the content itself was nothing special." (Read more ninjas stories.)