Grinding corn, making tortillas.

It turns out that corn, maize, that great staple, is not so nutritious on its own, potentially causing a vitamin-deficiency disease called pellagra. But ancient people noticed that soaking it in strongly-alkaline lime water (pH 11) made it more satisfying (or, at least, more easily processed for dinner), thus bypassing a dietary problem. And combining corn with beans made a more complete protein.  Here’s a fascinating peek at some food history:   http://en.wikipedia.org/wiki/Nixtamalization

Corn crib full of ear corn in a Maya village north of Piste