New Jersey, the 1970s. A girl is checking a long multiplication problem with a trick called casting out nines. Karen Smith loves it. Math feels like a puzzle. Then her class switches to base 8. The trick fails. Most students move on. Karen cannot. She keeps at it until she finds that in base 8 you cast out sevens, not nines. She still wants to know why. That question stays with her for years. In high school she meets modular arithmetic, the tool that explains it, and writes a proof that the method works in any base. On her own she starts imagining a geometry where parallel lines meet at infinity. She does not know mathematicians have studied it for centuries. It is called the projective plane. A teacher, Mr. Drinfeld, sees what she is doing and opens an extra class for a few seniors. One of the books is number theory. Years later she says, “I loved it. It definitely made me a mathematician.” At Princeton, her freshman calculus professor is Charles Fefferman, who will win the Fields Medal in 1978. He tells her something she has never considered. She could make mathematics her career. She graduates in 1987, teaches high school for a year, then learns a math PhD can come with full support. In 1988 she moves to Ann Arbor. Her mentor, Mel Hochster, introduces her to tight closure, a new idea that becomes the center of her work. That fall she meets Finnish mathematician Juha Heinonen. They marry in 1991. She earns her PhD in 1993. The thesis is published the next year in Inventiones Mathematicae. She goes to Purdue, then MIT, then back to Michigan as a professor. A 20-hour lecture course she gives in Finland in 1996 becomes a textbook in 2000. In 2001, at 35, the American Mathematical Society gives her the Ruth Lyttle Satter Prize and calls her a world leader in tight closure. She and Juha raise three children, including twins. In October 2007 Juha dies of kidney cancer. The twins are four. Karen is a single mother of three with a research career still in front of her. She does not quit. The children keep speaking Finnish. Every summer they go to Finland. Every week they bake Finnish rye bread. She keeps teaching, proving theorems, and training students. By 2017 she has written with 32 coauthors and guided 20 students through their PhDs. In 2019 she is elected to the National Academy of Sciences. In 2021 she is elected to the American Academy of Arts and Sciences. Today she chairs the mathematics department at the University of Michigan. It started with a trick that stopped working in middle school. She did not accept the failure. She asked why until she had the answer. Then someone told her mathematics could be her life’s work.