Holt Science will help you to explore some topics in science. There are five main parts. The first part is called Beginnings. This section explains how scientists investigate and interpret the events of the natural world. Following this introduction are four main parts, or units. Unit 1 is devoted to the study of the living world. The next unit deals with the forces that have shaped our planet. Unit 3 concentrates on the study of electricity, magnetism, and heat. The last unit explores the earth's position in the solar system and the universe beyond. Each of these units is divided into chapters. A review at the end of each chapter will help you to remember what you have learned. Each chapter is divided into several lessons. These objectives tell you what you will learn as you study the lesson. You will also have a chance to perform an activity in each lesson. This activity will help you better to understand the main idea of the lesson. You will find many interesting and helpful photographs and diagrams in each lesson. Science words that may be new to you are printed in boldface type. Their definitions will be found alongside in the margin. Each lesson ends with a short summary of the important ideas of the lesson. A few questions following the summary will help you to check that you have accomplished the objectives of the lesson. Most chapters contain an optional laboratory exercise. In a scientific laboratory, scientists observe what happens in nature. Your classroom can become a laboratory in which you can observe and begin to understand much about science.