Consider an example from the world of sports. The Dallas Cowboys score two quick touchdowns against the San Francisco 49ers and lead, 14–0. They are driving for a third touchdown, which will seemingly...

Consider an example from the world of sports. The Dallas Cowboys score two quick touchdowns against the San Francisco 49ers and lead, 14–0. They are driving for a third touchdown, which will seemingly turn the game into a rout, when disaster strikes: A pass is intercepted and returned for a touchdown. The 49ers appear revitalized, and the announcers proclaim that the “momentum” of the game has changed. Can momentum be seen or measured? Why do announcers and fans fi nd this construct useful?