Asperger syndrome can involve an intense and obsessive level of focus on things of interest. For example, one person might be obsessed with 1950s professional wrestling, another with national anthems of African dictatorships, and another with building models out of matchsticks. Particularly common interests are means of transport (such as trains), computers, and dinosaurs. Note that all of these last items are normal interests in ordinary children; the difference in Asperger children is the unusual intensity of their interest.
Sometimes these interests are lifelong; in other cases, they change at unpredictable intervals. In either case, there are normally one or two at any given time. In pursuit of these interests, the person with Asperger's often manifests extremely sophisticated reason, an almost obsessive focus, and great memory for apparently trivial facts (occasionally even eidetic memory). Hans Asperger called his young patients "little professors" because he thought his thirteen-year-old patients had as comprehensive and nuanced an understanding of their field of interest as university professors.
Some clinicians would not entirely agree with this description; for example, Wing and Gillberg both argue that there is often more rote memorization than real understanding of these areas of interest, despite occasional appearances to the contrary. However, such a limitation is not required for diagnosis, even under Gillberg's criteria.