This group of foods comprises a great number of highly complex substances, usually of vegetable sources, but whether directly from vegetables or from animals, they all owe their complex composition to plant life, for, as shown above, all organic animal foods come either directly or indirectly from the vegetable kingdom. These complex vegetable substances are built up step by step from simple mineral substances through the use of the sun's energy by the green pigment of plants. This process has been technically called photosynthesis. In this process of photosynthesis the energy of the sun's light and heat becomes latent, or in some mysterious way so linked with the matter that is being constructed that it is not apparent - that is, it is not active - so we call it latent. If one pumps a tank full of water to the top of a high building, that water, because of its elevated position, is capable not only of distribution to all parts of the building having a lower level than that of the tank, but it might incidentally run water motors, thus accomplishing a great deal of mechanical work. If all of the energy liberated by this water as it flows back to the level of the pump were summed up, it would equal the amount of energy required to lift it to the elevated position. In a similar way, the building up of complex substances by the plants lifts them into an elevated position in which the energy remains latent until it is later released. When an animal takes organic food into its system, the purpose is for the most part to procure the energy latent in that food. When the food is oxidized, the energy is released in the form of heat, muscular movement, gland activity, and nervous activity. In a similar way, the use of fuel in an engine releases on oxidation the heat which sets in motion the mechanical parts.