Question

Esquivel knows the story of maiz is a myth, so why does she warn her daughter not to step on a kernel of corn?

Answer 1

I'd say that because in their culture maize is a symbol of fertility, and that the kernels are part of a ritual. 

Answer 2

When Esquivel was a little girl, her family would tell her not to step on a kernel of corn. This is because corn was sacred.

When Esquivel grew up, she realized that this story was a myth, and that nothing bad would happen if she stepped on the kernel. However, she still taught the myth to her daughter because she saw it as a way to maintaining oral and cultural traditions alive.