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Every year more than three million miles of dental floss are sold in the United States. That’s a lot, but the total should be much higher, since that amount works out to only about one flossing per week for the average American. Dentists recommend flossing at least once a day to prevent gum disease, which robs more adults of their teeth than all other oral problems combined. A recent study reported that almost half of Americans feel guilty because they don’t floss their teeth, and 32 percent feel even more guilty for lying to their dentists about it.

In a recent issue, one of our authors recalled the atom-bomb drills that his grade school held. Your columnist is barely old enough to have experienced a few of these in the late 1960s. By that time even young children had some inkling of how terribly destructive atomic bombs were, and every time we had a drill, someone was sure to express skepticism about whether kneeling with our heads against the wall would be of any use against a nuclear attack.

Polymerase is the chemical that a cell uses to replicate its DNA when it divides. The polymerase attaches to a single strand of DNA after the molecule splits and builds a complementary strand to re-create the double helix. Polymerase chain reaction (PCR) takes advantage of this mechanism to make many copies of a small DNA section.

In forensic analysis, the polymerase chain reaction makes identical copies of a specific short section of DNA. It targets and multiplies a repeating section of DNA, making it easy to measure. After extracting the small sample of DNA needed, the technician places it in a test tube with the cocktail of chemicals used for the steps below.

This is the year for halls of fame to play catch-up. This summer the National Baseball Hall of Fame inducted 17 previously neglected figures from the Negro Leagues. And in May the National Inventors Hall of Fame, in Akron, Ohio, inducted a record 78 members, including 57 long-overlooked inventors, mostly from the nineteenth century.

In February 2006 Britney Spears earned worldwide opprobrium after photographs surfaced of her driving a car while holding her infant son in her lap. Spears’s failure to strap the tot into a child car seat provoked consternation reminiscent of that which had greeted Michael Jackson three years earlier when he dangled his baby over a balcony railing. The Los Angeles County sheriff’s office consulted with local child-welfare officials before deciding not to press charges.

Restriction fragment length polymorphism analysis is designed to measure the length of specific segments of DNA that contain repeated patterns of base pairs. The cells containing DNA may be from a crime scene or obtained from white blood cells or a mouth swab from inside the cheek of a suspect.

Readers of this column have been known to complain that it relies too heavily on bridges. But there are some things you just can’t get too much of, and bridges are one of them. Environmentalists often speak of “charismatic megafauna”—large, cute animals like pandas and dolphins that are indispensable in raising funds. When it comes to historic technology, bridges are charismatic megafauna, and the species with the greatest “Aaawwww!” factor is covered bridges.

In the 1970s researchers discovered enzymes in bacteria that attach to any DNA molecule when they find specific sequences of base pairs. They reliably slice the molecule between two of those pairs. For example, the restriction enzyme EcoR1 (obtained from Escherichia coli bacteria) attaches to DNA when it detects the sequence GAATTC and cuts between the G and the A.

A couple of years back I was wandering around at the Kentucky Bourbon Festival, held in early September in Bardstown, Kentucky, when I spotted a man fashioning a barrel from what appeared to be some untidy scraps of wood. I felt a little sorry for him. Most festivalgoers had come to sample bourbon. The barrel man had no bourbon to offer, so few people were stopping to chat.

(1906-2005) Physicist, Nobel laureate, head of the Theoretical Division at wartime Los Alamos. His solar research led to speculations on the feasibility of a hydrogen bomb.

(1901-1985) Professor of French literature at the University of California, Berkeley, Communist, and close friend of Oppenheimer. In 1942 Chevalier asked Oppenheimer to help transfer information about the bomb to the Soviet Union. Oppenheimer refused, but the incident caused him much trouble later.

THE PICTURE AT RIGHT WAS TAKEN ON THE MORN ing of May 25, 1953, at the U.S. Atomic Energy Commission’s Nevada Proving Ground (now the Nevada Test Site). It shows the scene less than a minute after the Army’s gigantic “atomic cannon” made its first and only firing of a nuclear-tipped artillery round. The gun itself was a conventional cannon, but it had to be extraordinarily big and built with unique features to accommodate its special projectile.