Modeling can be demanding and stressful; models must be ready to work on short notice, with unpredictable, varying hours for each job. Part-time work with periods of unemployment is common. There are no formal education requirements for becoming a model, and modeling schools do not guarantee employment. Models maintain a portfolio of their work for job applications, and may also use social media to build a following and increase exposure. Most modeling agencies accept photos from applicants and interview those who meet the agency’s specifications for appearance, including height, weight, and clothing size. Agencies also host “open calls,” for prospective models to walk in to meet directly with agents and clients.