Kids. understanding of the world is shaped a whole lot more than you may think by what you say and how you say it.
Most parents today would think the old saying "Children should be seen and not heard" belonged in the past. The same is probably true for "Walls have Ears" but many parents and teachers just don't realise the influence they have on how a child's future will develop.
They do hear things we may not intend them to hear, they do need to speak up for themselves as they work at becoming independent human beings and managing all this without constantly beating yourself up and being frightened to say "NO" when that really is what you know you should say or to gently curb petulant attention seeking behaviour can just make you a bag of nerves.