And the Lord said, Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat: Luke 22:31
The process of threshing and winnowing grain contains elements of being crushed to break off hulls, then being repeatedly thrown into the air and wind tossed from high atop a threshing floor. Entire families take part in the harvesting work. It is a strenuous and repetitive process that separates the chaff from the grain.
Below, James Neil describes the continuing actions needed to further sift and prepare the grain for use:
"Wheat in Palestine, after it has passed through the exceedingly simple processes of threshing and winnowing on the open-air threshing-floors, comes into the market in a very dirty state. Dust, pebbles, short pieces of crushed straw, damaged grains, barley, and the seeds of wild plants (the bitter and poisonous “tares”), mingle largely in the measures poured into your sack by the professional measurer.
But they never trouble much about that in Palestine. Provided there is good wheat, and a due proportion of it, they take but little account of whatever else the sack may contain. And the reason is this — that the separation of the refuse from the grain is a regular part of household work at the time of breadmaking. The process, with which I became very familiar, was always carried on in the open courtyard of our parsonage home on Mount Zion.
The woman servant seats herself on the ground with her feet spread widely apart, taking in her hands a large but shallow sieve some two and a half feet across. Having placed a small quantity of wheat in the sieve, she commences by giving it some six or seven sharp shakes, so as to bring the chaff and short pieces of crushed straw to the surface, the greater part of which she removes with her hands.
After this the main part of the work begins, which is done with much skill. Holding the sieve in a slanting position, she jerks it up and down for a length of time, blowing across the top of it all the while with great force. In a word, she turns herself into a regular winnowing machine!
Three results follow. In the first place the dust, earth, small seeds, and small, imperfect grains of wheat, etc., fall away through the meshes of the sieve. Secondly, by means of the vigorous blowing, any crushed straw, chaff, and such-like light refuse is either blown away to the ground, or else collected in that part of the seive which is furthest from her. Thirdly, the good wheat goes together in one heap about the centre of the sieve, while the tiny stones or pebbles are brought into a separate little pile on that part of it which is nearest to her chest. The pebbles, chaff, and crushed straw thus cleverly removed from the corn, mainly by the angle at which the sieve is held and the way in which it is jerked up and down, are then taken out with her hands. Finally, setting the sieve down upon her lap, she carefully picks out with her fingers any slight impurities which may yet remain, and the elaborate and searching process of Biblical sifting is complete."
It seems that the Lord was warning Peter of coming persecution and suffering. But that process of being sifted would lead not to destruction (as Satan hoped) but would enable Peter to be increasingly purified and eventually sanctified as he was willing to endure to the end and complete the process. I'm guessing that these words can also be likened unto ourselves.