There are many things that people say and do that make us question their common sense and intelligence in general. Just a few that I have noticed are:

Basic Manners- People seem to lack the skill that come with being a decent human being. From saying please and thank you to simply being able to say "hello" and "goodbye", many just do not seem to get how they should act towards others.

School Work- It is pretty clear that a big part of doing well in school for the majority of students is actually doing the required work. One would think that it would be understood that if homework and classwork is not done, the grades received will reflect that. However, the amount of people who still complain about their grades and their teachers and then proceed to do absolutely nothing about it is astounding. 

Driving- As a high school student with a little over a year of experience, my driving skills are definitely not incredible. Yet even I am surprised by the amount of people who do not know things like how to use turn signals and/or make incredibly abrupt stops.

On a less serious note, some stupid things I have heard (and may or may not have said myself) include:

Saying 111= "One hundred and ten one"

*Forgets the word chicken so decides to call a chicken a dog*

"What is a mathematical concept and a tasty desert?" "Subtraction?"