You shouldn't tell someone they are not intelligent.
Megan is assigned to work with a colleague on a project at work and her colleague doesn't quite understand the task.
Megan wants to motivate her colleague to try harder.
Megan gives her colleague some constructive advice relevant to the project.
Megan questions their intelligence thinking that this will light a fire under them.
Megan's colleague becomes more confident in the task and his performance improves.
Megan's colleague feels discouraged and it affects his performance in a bad way.