"I went to every art supply store in town, and every hardware store, too," he said.   "Nobody had an 18-inch ruler.   So ... I bought a yardstick and sawed it in half."

And that, of course, was the professor's "challenge" all along ... he didn't want them to find an 18-inch ruler.   He wanted them to solve the problem of it not existing.   Only one student figured it out.