What do you breathe in and out?

Answer 1

You breathe in air, which is comprised mostly of nitrogen (though the oxygen is what you're after).

The oxygen is absorbed into the blood, and stored in red blood cells, which contain hemoglobin.

The oxygen in your lungs is thus replaced mainly by carbon dioxide, which is a metabolic waste product, and this is what you breathe out.

Answer 2

Oxygen in and carbon dioxide out. Plants are reverse....