INTRODUCTION 5 



The clouds move with the air. Heat draws water up into 

 the air, by means of the process we call evaporation. Light, 

 shining on the green plants that cover the land, enables 

 them to make the food on which animals live. Water, 

 moving in clouds and falling as rain, enables the plants to 

 live in the soil in which they are rooted. From the soil 

 and from the air they get the substances that they make 

 into food. Here we see one of the great partnerships of 

 nature. Heat and light and air and water and soil and 

 green plants are the members of this partnership, and the 

 food that sustains our lives is one of the results of their 

 work together. 



Of the number of things absolutely necessary to life, 

 food is usually the most difficult to obtain. Air to breathe, 

 water to drink, and enough heat to keep life going can 

 usually be. obtained by all living things without much 

 work. But with food it is different. Nature requires that 

 all living things work for their food. Though now you 

 may be getting your food for nothing, somebody else has 

 certainly had to work to provide it for you. Of the money 

 that we spend for really necessary things, more is spent 

 for food than for anything else. It takes a great deal of 

 work to cultivate the plants and to raise the animals that 

 are the sources of human food. If we do not do food- 

 producing work ourselves, we must pay others to do it 

 for us. So, as we study the working together of the 

 forces of nature, it will be interesting to see how these 

 things are related to the production of food. 



Let us go over this matter once more to get the picture 

 dear. Think of the heat of the sun that starts the pro- 

 cession of changes; it starts them by causing movements 

 of both air and water. Think then of the water falling as 



