Dear Dr. Boli: While I was being driven by a friend I was visiting in Pittsburgh, she said: “You have to watch out for drivers who are driving the other straight ahead.” I didn’t see what she was referring to, because my eyes were closed in terror. What could she have meant by it? —Sincerely, A Passenger in Pittsburgh.

Dear Sir or Madam: Most cities can be mapped with a fair degree of accuracy in two dimensions, which is why maps of other cities can be sold on paper. Maps of Pittsburgh are also sold on paper, but only to gullible tourists who expect a map to be flat. The peculiar topography of the city makes it quite possible for two streets to occupy the same space on a two-dimensional map, but one above and the other below. That gives us three dimensions at the minimum for an accurate map; but then there are many roads or lanes that go in different directions at different times, which adds a fourth dimension; and so on. It was commonly believed among topographers that ten dimensions were necessary for a completely accurate description of any intersection in Pittsburgh, but recent work in string theory suggests that an eleventh dimension may be necessary as well. In any case, if you plan another visit, you may wish to review the latest developments in particle physics beforehand.