This book is a new take on the I Spy or Where's Waldo books. In this book, two photographs are taken of each large machine. The second photo contains differences which require the reader to look closely at details in both photos. The skill levels range from easy, with 6 differences, to extreme, with 12 differences. Answers are circled on a small version of the second photograph found in the back of the book. Two other books on related themes are mentioned in the back. Facthound internet site is included as a safe and fun site for young readers. Other puzzles can be found.