In 1964, DuPont chemist Stephanie Kwolek produced a polymer solution that looked like a failure. It was thin, cloudy, and reminded her of buttermilk, the kind of mixture standard practice sent down the drain. She kept it. Stirring revealed an odd opalescence, a sign that the rigid aramid molecules had lined up in a liquid crystal state. The technician who ran the spinneret, Charles Smullen, refused to test it for fear of clogging his equipment, until she talked him into it. The fiber that emerged would not snap.

This episode follows Kwolek from New Kensington, Pennsylvania, where she was born in 1923 to Polish immigrant parents, to a 40-year career at DuPont that began as a temporary job to pay for medical school. A naturalist father taught her careful observation and a seamstress mother taught her how fibers bear a load. Together those lessons led to Kevlar, a material five times stronger than steel by weight that DuPont introduced commercially in 1971.

• At her 1946 DuPont interview in Buffalo, she demanded an immediate answer, citing a competing offer, and the offer letter was dictated on the spot.
• Her nylon rope trick showed that polymer could form at room temperature where two unmixable liquids meet, letting a continuous thread be pulled from a beaker.
• Kevlar’s strength comes from molecules aligned in parallel, like uncooked spaghetti in a bundle instead of a tangled cooked bowl.
• She signed the patent over to DuPont and earned a salary and pension while the company made billions from more than 200 applications.
• The one millionth bullet-resistant vest made with Kevlar was sold the week she died in 2014 at age 90, after retirement years spent teaching chemistry to schoolchildren.