Damn title character limits. I just had the weirdest thought while rearranging my computer's case fans. I put my hand in front of one of them and felt a light breeze of air, telling me it was working. This is due to the local cooling produced where the fan's airstream comes in contact with the skin. Since the air being blown is at a lower temperature than the human body, that local skin cooling triggers my sensory neurons, my skin feels slightly cold and thus I know there's air flowing against it. But what if the air were the exact same temperature as my skin? No heat transfer would occur and my neurons would not be activated. Thus, I can only think of two other mechanisms by which I could perceive the fan if I were not looking at it nor hearing it: perhaps the moving air slightly disturbs the hairs on my skin, which trigger sensory neurons of their own, OR the airstream would be strong enough to actually push my skin. The latter case is obviously not true with 120 mm case fans. Assuming my hair sensitivity theory is correct, if the fan were functioning in a low enough setting so that the airstream's speed did not disturb the hairs, it stands to reason that the fan would be completely undetectable if the air temperature were the same as my body's. I would not be able to feel it blowing air at all. It's quite a perturbing thought, no?