Plants use a process called photosynthesis to convert sunlight into food. During photosynthesis, plants absorb sunlight through pigments like chlorophyll in their leaves. This sunlight is used to convert carbon dioxide from the air and water from the soil into glucose or sugar, which serves as the plant's food source. The byproduct of this process is oxygen, which is released into the air as a waste product. In summary, plants use sunlight to power the chemical reactions that produce glucose, their primary source of energy.