Don't misclassify an employee as an unpaid intern. In a nutshell, if an internship benefits only the intern and not the person who is providing training, the intern is generally not classified as an employee. The Department of Labor emphasizes that internships in the private sector will most often be viewed as employment unless the internship meets a test of exclusion. The Wage and Hour Division of the Department of Labor has issued specific criteria that should be used to determine whether an intern is a trainee or employee who must be paid minimum wage and overtime if he or she provides services to a private-sector employer.
See the article, "When is an Intern an Employee?" for the requirements of an internship.