Grant begins with an anecdote about a group of elite firefighters in the 1940s. In 1949, 15 smokejumpers parachuted into Mann Gulch to extinguish a forest fire started by lightning the day before. The fire soon became uncontainable, and the foreman, Wagner Dodge, ordered them to retreat. When escape seemed unlikely, Dodge began burning the grass in front of them and called his crew toward his fire. Thinking he was crazy, they continued trying to outrun the fire.

What his crew didn’t realize was that Dodge had created an escape fire. He burned out a small area, which deprived the fire of fuel, and huddled in the clearing while the fire passed over him. Of the remaining smokejumpers, only two managed to outrun the fire. Dodge hadn’t been taught how to create an escape fire—it was pure improvisation. The method was so strange to his crew that they were unable to rethink their assumptions.

Grant argues that Dodge survived due to his mental fitness. He distinguishes this from intelligence, suggesting that mental fitness includes not only the ability to think and learn, but the ability to rethink and unlearn. For example, common wisdom suggests that changing your answers on a test will likely hurt your score; however, studies show that students are more likely to change their answers from wrong to right.