The Mark I's successor, Mark II, was run without air-conditioning in a room with open windows. One hot summer day in 1945, the Mark II stopped. Grace Hopper and her coworkers investigated and found that the source of the trouble was a moth that had been killed by a relay closing. The dead moth was removed and the incident was recorded in the logbook. Before long, Aiken came in to ask whether they were "making any numbers"—which was his phrase for computing. His team explained that they were "debugging" the Mark II. The word "bug" had been in existence for more than a century to describe mechanical system failures, appearing first in the writings of Thomas Edison, but this was allegedly the first use of "debugging" as a computing term.