Yes," I say to the boy studying rattlesnakes, "that is a cool search string (sidewinder-games-cycle-computer-baseball) ... what if we simplify it?" After a day filled with Google operators, tricky citation questions, and some final editing of manuscripts for this issue, I'm ready to relax. Determined to scan today's morning paper before tomorrow morning, I pour a mug of tea and spread out Monday's front page. My lethargy evaporates, hooked by "Code3," part II of a special report on San Francisco's emergency response time to 911 calls. I shuffle back through the pile to Sunday's unread paper and locate part I. Deep in the article, a half-page map (on previous page) of the city's emergency-response districts is displayed and labeled with the percentage of life-threatening emergencies for which medical help did not arrive within 6 1/2 minutes, the city's goal from call to arrival. The map's key includes fire stations, ambulance locations (some of which are at fire stations), and hospital emergency rooms. Scanning and decoding (What's on this map?) gives way to scrutiny and analysis (What's their relationship? What does it mean?).

I begin wondering what characteristics the lower-percentage districts, those with the shortest response times, have in common. I count the number of fire stations and ambulances in each district--no pattern there. I see that the districts vary in size--why? Then I look for districts of equal size and find two. The lowest percentage district and one of the highest are almost the same size--and they're adjacent. What makes one more successful? I've driven through both but don't remember anything--traffic snarls or construction detours--that would distinguish them. Arteries are ghostly visible and Google real-time live traffic and street view maps are no help--too narrow a slice of time and place. I count the fire stations and ambulances again. Then I count the hospitals. The successful district has three, the other none.

I review what I know and how I know it. Where's it leading? Why would hospitals impact the response time? Are the chances higher that an ambulance would be available more often at the emergency room? But what about the ambulances based at fire stations? I recount the fire stations by district. Why don't districts with more fire stations have a similar advantage?

Maybe population density varies--but by how much? Does the number of calls in each district correspond to population density? Or higher crime rates? Perhaps there's a correlation between better response rates and wealthier districts? That gets me wondering about the emergency-response rate in my city, Palo Alto. Questions reflect interests and assumptions as well as curiosity; it's important to acknowledge that your own point of view shifts you toward certain information.

What influences the cartographer's choices? Why does the map display the percentages of calls that failed to arrive in 6 1/2 minutes rather than the success rates? Why isn't population density listed for each district?

Why this selection and presentation--and by whom? The longer you look at a map, the more likely that you...