It's seldom denied that critical thinking is an important skill to develop, but "fun" isn't usually an adjective associated with logic, syllogisms and informal fallacies. The James Madison Critical Thinking Course by two philosophy professors may change even the most reluctant reader's mind, however, as it guides high school and adult students through the main points of critical thinking using fictional crime scene investigations to introduce and practice concepts.
Eight chapters are each based on a particular aspect of critical thinking. Students begin with the basics (what critical thinking is, claims and statements, etc.) and progress to arguments, syllogisms and fallacies. Everything included in a typical introductory logic course is here, with thorough treatments supplemented by plenty of exercises—the constant quizzes, problems and written assignments help cement information and offer a practical perspective to a subject too-often overly abstract.
The large text is a consumable workbook with tear-out pages to facilitate completion of exercises. Exercises form the bulk of the text (there are no illustrations), with exactly as much instruction and description as necessary. Where many logic courses focus on endless examples, authors O'Meara and Flage contend that allowing students to put principles to use is a better teaching method than mere repetition of information. These aren't just boring exercises, either—the CSI theme makes them genuinely fun to complete. An Instruction/Answer Guide simply offers solutions to exercises.
Concepts include both formal and informal logic. In the introduction the authors stress the need to continue to hone one's skills after completing this book, though it will provide an excellent grounding. If your high school kids (or you) don't have much experience with logic this is a great place to start—it isn't overwhelming (despite its length), but it's not logic-lite. Starting at the beginning, students progress in an orderly manner through various stages of the critical thinking process, guided by clear text that is largely jargon-free.
There are better logic courses out there, but not of the cold-turkey introduction variety. For those unfamiliar with the basic terminology, concepts and systems of good reasoning, the James Madison Critical Thinking Course is hard to beat. It doesn't talk down to older students, but solving mini-mysteries adds enough of a fun element to help the real content go down easier. For a more thorough and decidedly more difficult follow-up, we suggest Canon Press Logic; some of the material will overlap, but it would be a good idea to start with Introductory before moving on to Intermediate.