Why is it inaccurate to say that we breathe in oxygen and we breathe out carbon dioxide?

It's inaccurate because we actually breathe in a mixture of gases, with oxygen being the most abundant at about 21%. When we breathe out, the air we exhale still contains a significant amount of oxygen along with other gases like nitrogen and some carbon dioxide. The exchange of gases occurs in the lungs, where oxygen is taken up by the blood and carbon dioxide is released.