Saving seeds is an art, however. And it isn't something the grower can just decide to do after the crop is picked. It begins all the way back at planting time, with the way garden space is allotted and various strains of the same crop are separated. You can even do several varieties if you keep them separate. Wind or insects can cross the pollen, and what you end up with as seed won't grow the same plant as the parent plant.
Tomatoes do almost no cross-pollinating, but some varieties of corn have to be kept a mile apart.
I'm not even going to try corn. Even in this age of commercial seed vendors, the centuries-old practice of gathering and swapping seeds has persisted. In some regions, seed swaps retain an old-fashioned character as social events, rather like quilting bees. The standard time is January, when snowbound fields can't be worked, but farmers and horticulturists must get seed for the new year's planting season. The local seed swap at Hampshire College in Amherst, Mass., promises to become an annual event for the foreseeable future.