A tutor is an instructor who gives private lessons. The most famous example of a tutor is Aristotle, who tutored Alexander the Great. A tutor is not to be confused with a teacher who is employed in the education of groups. To tutor (as a verb) is to perform the functions of a tutor. Shadow education is a pejorative name for private supplementary tutoring that is offered outside the mainstream education system. Private tutoring can help high achievers reach new levels. It can also help slow learners keep up with their peers. In order to provide children with educational advantages, the use of private tutors is spreading and intensifying.