Critical thinking skills allow you to analyze a Primary Source to gather as much information as possible from it. Using your own experiences and prior knowledge of a historical event, much of which you develop when you read books and other secondary sources about a topic, you can raise questions and problems about the sources that help you determine whether or not the source is useful for your project.
Critical thinking skills recognize that human beings have limited perception -- we just can't see and comprehend everything that happens. Understanding this fundamental fact is essential when considering your own or instructing students in the use of critical thinking skills with primary sources.