This textbook is a volume in the Pathways in Science series, designed to meet the demands of the modern tendency to introduce science as an essential part of the elementary-school curriculum by presenting a course of carefully graded problems in science for Grades I to VI. Each volume of the series has been organized about a number of units. These units present a series of problems, each of which offers a real challenge to children. Sufficient information is given in the text to lead to a satisfactory solution of the problem and to an understanding of the essential meanings that are involved in the presentation.