Windsocks are intentionally made with one end being narrower than the other. When air is forced through the larger opening and out the narrower opening, it has to work hard to push its way out because the opening that it came in through was wider than the opening through which it is exiting. This forces the windsock to inflate as the air pushes itself through the narrow end. How much the windsock is inflated, tells the onlooker how hard the wind is blowing.