I once had a student that was having a very difficult time with flexible thinking. He would learn a fact from school or from a book, and then when something else had that quality he would become very confused. Teaching him new things was a constant struggle. I remember specifically that he learned monkeys were primates. Then in the next lesson his teacher taught that humans were primates. The student would not accept that fact - only monkeys could be primate in his minds. This type of rigid thinking can be a challenge for many individuals with autism.
To help create more flexible thinking, visual organizers like the included Venn diagram can be helpful. Simply put two key words in the top two boxes, and the student needs to come up with three things are unique to each, and three things that are the same. When I was teaching in my classroom, I would use these types of sheets daily to encourage more abstract, flexible thinking. You can pick fairly obvious key words at first, and even include photos, then move to more abstract concepts as the student gets better!