What are marginal benefits?
For example, I work in the office 8 hours everyday. One day, I decide to work one extra hour，because I believe this one additional hour working will bring me more additional benefits than costs. The additional benefits can be a higher quality lesson for my students or weekend's work can be finished earlier so I can have a free weekend. The marginal (additional) cost can be one hour less to be with my family, or late dinner which can cause stomach.