Even though Esquivel knows the story of maiz is a myth, she nevertheless warns her daughter not to step on a kernel of corn because the corn represents fertility and the kernels are part of the ritual. It would be wrong, or perhaps even sinful to step on such an important symbol of their culture which is why Esquivel doesn't want her daughter to be in some kind of danger because of such a reckless act.
Esquivel knows the story of maiz is a myth but she still warns her daughter not to step on a kernel of corn because it is a symbol of life, fertility and prosperity.
