The most distinguishing symptom of Asperger syndrome (AS) is a child's obsessive interest in a single object or topic to the exclusion of any other. Children with AS have become experts on vacuum cleaners, makes and models of cars, and even objects as odd as deep fat fryers. Children with AS want to know everything about their topic of interest and their conversations with others are limited to this topic. Their expertise, high level of vocabulary, and formal speech patterns make them seem like little professors.
Children with AS will gather enormous factual information about their favorite subject and talk incessantly about it. However, the conversation may appear as a random collection of facts or statistics, with no point or conclusion.
Their speech may be marked by a lack of rhythm, an odd inflection, or a monotone pitch. For example, they will have to be reminded to talk softly every time they enter a library or a movie theatre.
Unlike the severe withdrawal from the rest of the world that is characteristic of autism, children with AS are isolated because of their poor social skills and narrow interests. In fact, they may approach other people, but make normal conversation impossible by inappropriate or eccentric behavior, or by wanting only to talk about their singular interest.
Children with AS usually have a history of developmental delays in motor skills, such as pedaling a bike, catching a ball, or climbing outdoor play equipment. They are often awkward and poorly coordinated with a walk that can appear either stilted or bouncy.
Many children with AS are highly active in early childhood, and then develop anxiety or depression in young adulthood.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.