Zitkala-Sa, also known as Gertrude Simmons Bonnin, was a Native American writer and activist who wrote extensively about the experiences of Native Americans in the late 19th and early 20th centuries. In her autobiography, "American Indian Stories," Zitkala-Sa describes her time at a Native American boarding school, where she encountered a strict set of rules and regulations that governed every aspect of the students' lives, including their eating habits.

The term "Eating By Formula" refers to the rigid and prescribed set of rules that the students at the boarding school had to follow when it came to eating. Zitkala-Sa explains that the students were expected to adhere to a specific eating decorum, which involved strict rules about how to sit, how to hold their utensils, and how to eat. This rigid set of rules left no room for individuality or personal expression, and the students were expected to conform to this set pattern of eating.

The purpose behind this strict eating decorum was to enforce a sense of discipline and control over the students. By dictating how the students should eat and behave during mealtimes, the school authorities were attempting to erase their Native American identity and replace it with a more Euro-American one. The students were taught that their traditional ways of eating, which may have involved communal eating or using their hands, were uncivilized and backward. Instead, they were expected to adopt the manners and customs of the dominant white culture.

This emphasis on conformity and assimilation was a common theme in Native American boarding schools during this time period. The schools were designed to strip Indigenous children of their cultural heritage and force them to adopt a Westernized way of life. Eating By Formula was just one example of the many ways in which the students' individuality and cultural identity were suppressed.

Overall, Zitkala-Sa's use of the term "Eating By Formula" highlights the oppressive nature of the boarding school system and the ways in which Native American children were forced to abandon their cultural traditions in favor of assimilation into white society.