That's a great question. Plants use photosynthesis to create the sugars they use for energy. Plants are called "primary producers" because they are the base of the food chain. All organisms who cannot convert chemical or light to usable energy depend upon organisms that can, like plants. Photosynthesis takes carbon dioxide (a gas in the atmosphere) and makes sugars using that carbon. Plants use these sugars for energy, just like people. Photosynthesis also releases oxygen that we breathe. Plants use oxygen too, to metabolize the sugars they create.