Chlorophyll bears a striking chemical similarity to hemoglobin, the vital component of blood. Chlorophyll's phorphyrin structure has magnesium as its central metallic element; hemoglobin has iron. When an animal eats grass, a metathesis occurs in its intestinal villi, transforming a large amount of magnesium into iron. Metathesizing chlorophyll creates increased hemoglobin in the blood, which is why vegetarian animals can maintain life by eating only green grass. That's also why some people call chlorophyll "green blood."
Chlorophyll is primarily found in leaves and is responsible for a plant's ability to make food through photosynthesis. It is responsible for transforming carbon dioxide in the air to oxygen and it uses the energy of the sun to manufacture nourishment for the plant.
According to a 1999 study by the Eighth Asia Nutrition Study Board, chlorophyll also helps prevent cancer by defending against harmful variations in meat produced by cooking. But chlorophyll is also easily damaged by heat -- you'll notice that steamed vegetables produce greenish liquid, which contain the dissolved chlorophyll. That's why it's better to eat green vegetables raw.