Well, I think that by walls, people mean the invisible walls that we don't see. Example in school. Students go into their own groups of friends causing them to build up a wall against other people. Populars, geeks, etc. In school, maybe the fear of being dicriminated from everyone else keeps people from breaking through that wall. Or it could be that they are uncomfortable leaving their group.
Cell Walls.
the door and the walls protect the people inside
the door and the walls protect the people inside
If you are putting braces in walls it keeps them supported
Cats can hear termites moving around in walls. In fact, people can even detect the presence of these critters inside walls.
it is like a door it protects things or people inside it is like a door it protects things or people inside it is like a door it protects things or people inside
The type of air that keeps a beach ball inflated is kinetic energy.
Endometrium is found on the inside walls of the uterus.
"Muros" are outside walls, "paredes" are inside walls.
in the book inside the walls of troy the protagonist is the main character
The duration of Inside the Walls of Folsom Prison is 1.45 hours.
A borescope camera can be used to look inside walls.