That evening, Lena emailed her father, a brewer who struggled with kettle geometry. “Dad,” she wrote, “when you slant the bottom of your brew kettle to drain the trub, the optimal angle is the one where the derivative of the settling velocity equals the derivative of the flow rate. It’s a tangent line problem.”

She attached a photo of Simmons’ margin note, written in pencil by some long-dead student: “The tangent is not the end. It’s the direction.”

They stared. She pulled out Simmons. “Let me tell you a story about a Swiss guy named Euler…”

“You don’t need another problem set,” Emery said. “You need a story.”

Later that night, Lena couldn’t sleep. She read another gem: The Brachistochrone Problem . Johann Bernoulli bet his rivals that the fastest path between two points wasn’t a straight line, but an upside-down cycloid. Simmons wrote, “The curve of swiftest descent is the one on which a bead, sliding without friction, beats any rival—even the straight line.”

By semester’s end, Lena passed with a B+. But more importantly, she bought her own copy of Calculus Gems from a used bookstore. On the inside cover, she wrote: “For the next person who thinks calculus is just rules—read this. It’s actually a box of lightning in paper form.”