If you tell a teacher that a child is bright, the teacher will be more supportive, and shower all attention on the child. The child receiving this attention and basking in the teacher's belief learns more and is better in school. It does not matter if the child is actually bright.
All that matters is the teacher's confidence in the child. This is also true of managers and workers. This uniquely human phenomenon is called the Pygmalion Effect. It is a persistently held belief in another person such that the belief becomes a reality.