After threshing, Keshav and Rukhsar are helping their uncle separate the light husk from the wheat…

Why

Keshav is correct that in still air everything falls straight down — but that is exactly the problem. If husk and grain both land in the same heap, nothing has been separated. Winnowing works because the two parts differ in one property: husk is very light, grain is heavy. A gentle breeze barely moves the heavy grains, which drop close to the person, but it pushes the light husk sideways so it lands a little distance away in a separate pile. No breeze, no difference in where they land. That is also why, when the wind won't cooperate, people winnow in front of a fan. Test: drop a handful of rice mixed with torn bits of paper from a height, once in front of a table fan and once with the fan off, and compare the piles. Rukhsar is right: winnowing needs moving air, because the wind is what sorts the light husk from the heavy grain.