On Sun, 9 Aug 2015 16:17:04 -0500, Muggles wrote:


I used my knuckles to tap/knock on the wall the same as knocking on a
door, and I knocked on the wall firmly. I just couldn't hear the difference.

As shown. The difference is distinguished by a hollow or solid sound.
--
"No matter how cynical you become, it's never enough to keep up."
-- Lily Tomlin