Talk politics.

The biggest source of lead in the postwar era, it turns out, wasn't paint. It was leaded gasoline. And if you chart the rise and fall of atmospheric lead caused by the rise and fall of leaded gasoline consumption, you get a pretty simple upside-down U: Lead emissions from tailpipes rose steadily from the early '40s through the early '70s, nearly quadrupling over that period. Then, as unleaded gasoline began to replace leaded gasoline, emissions plummeted.

Intriguingly, violent crime rates followed the same upside-down U pattern. The only thing different was the time period: Crime rates rose dramatically in the '60s through the '80s, and then began dropping steadily starting in the early '90s. The two curves looked eerily identical, but were offset by about 20 years.

So Nevin dove in further, digging up detailed data on lead emissions and crime rates to see if the similarity of the curves was as good as it seemed. It turned out to be even better: In a 2000 paper (PDF) he concluded that if you add a lag time of 23 years, lead emissions from automobiles explain 90 percent of the variation in violent crime in America. Toddlers who ingested high levels of lead in the '40s and '50s really were more likely to become violent criminals in the '60s, '70s, and '80s.

(I emboldened.)

George Stephenson's Rocket, the first
commercially practical steam locomotive.

It is metaphorically satisfying to talk about threads being woven together when talking about cotton, but the thread that mattered to the Liverpool & Manchester Railway was made of iron: thirty miles of it, smelted, forged, and wrought in ironworks . . ., and laid down as rails between the two cities that were now producing, in their mundane way, more wealth in a year than the entire Roman Empire could in a century.

(William Rosen, The Most Powerful Idea in the World: A Story of Steam, Industry, and Invention, Random House, 2010, p. 303.)