Which two element are we know about from everyday use and how are they similar how are they different?

Oxygen (O2) and carbon dioxide (co2) are similar because they are interchanged between humans and plants where the alveoli in your lungs take in O2 and release co2, then the plant go through a process called cellular respiration where they extract what they need from carbohydrate and carbon dioxide and they release O2, now this repeats. That's why plants are necessary for human survival