A Japanese businessman's English lesson caused his flight to make an unscheduled stop in Chicago yesterday. He scribbled the phrase "suicide bomb" on a piece of paper. Another passenger saw it and alerted a flight attendant. The flight to Dayton, Ohio, returned to O'Hare Airport. The man was briefly questioned and released without charge. He explained that he's reading newspapers to learn English, and writes down words that he doesn't understand to look them up later. Police conducted a routine search of the plane and no bomb was found.