W.K. Kellogg left his brother's sanitarium to sell corn flakes

Battle Creek Sanitarium, run by Dr. John Harvey Kellogg, served a strict vegetarian diet to its patients, and a batch of wheat-berry dough left out overnight by accident turned out, once rolled the next day, to flake rather than form a solid sheet — creating what became flaked cereal. Sanitarium guests liked the flakes enough that after returning home, they began mailing in requests for more, an unprompted, repeated demand signal for a product that had only ever been served on-site.

W.K. Kellogg, John Harvey's younger brother and the sanitarium's business manager, saw that demand and wanted to sell the flakes commercially, sweetened to broaden their appeal beyond the sanitarium's health-focused clientele. John Harvey, committed to the flakes as a bland health food and unwilling to have the sanitarium's product commercialized or sweetened, forbade his brother from distributing it beyond existing patients — treating the mail-order requests as outside the sanitarium's mission rather than as evidence of a market.

Left the institution to act on unsolicited mail orders, treating them as validated demand and scaling with sugar and advertising.

Patients' unsolicited mail orders were free, repeated, self-selected proof of demand.

John Harvey's refusal to act on that signal left the opportunity open.

W.K. Kellogg, close to the signal but without authority, left to build a company around it.

Sweetening the product and aggressive advertising converted validated demand into mass volume.

When the person controlling a product ignores repeated unsolicited demand, act on it independently rather than let it go unaddressed.

By 1909, three years after founding, Kellogg's company was producing 120,000 cases of Corn Flakes a day, and a 1907 promotional campaign offering a free box to any woman who winked at her grocer reportedly increased New York City sales fifteenfold. The company was renamed Kellogg Toasted Corn Flake Company that same year and became the Kellogg Company in 1922.