This book is part of a series which aims to answer some of the most common questions that children ask about themselves and the world around them. It provides clear scientific explanations behind different aspects of life that are often taken for granted. Related and surprising facts are designed to stimulate interest and the imagination. Why do waterfalls fall? Why is the sea salty? How are mountains made? How old is the Earth? This book explains how different features have formed on the Earth's surface and includes details of the structures deep inside our planet. With experiments, illustrations and photographs, the book reveals some of the history of the Earth and looks at some of its many fascinating and unusual formations.