The hallmark feature of Autism is impaired social interaction.  A child's caregivers or teachers are the first to notice signs of Autism.  During infancy the baby shows signs of unresponsiveness towards people and may focus exclusively on one item and sit away from others for long periods.  A child may appear normal for the first year of life and then suddenly withdraw and become indifferent to social engagements.  They may fail to respond to their names and avoid eye contact.  They have difficulty interpreting what others are saying or thinking, they have problems with social cues. They can't pick up on facial expressions due to the lack of eye contact.  They lack empathy.  Many engage in repetitive movements such as rocking and twirling and self abusive behavior such as biting or head banging, and annoying repetitive sounds.Although they appear to be mentally impaired they are trainable and capable of living productive lives.