Reefers are used to carry two main kinds of cargo: chilled or fresh produce (primarily fruits and vegetables) and frozen foods (primarily meat, fish and dairy products). In both cases, the cargo has to be pre-cooled to the optimal carrying temperature, because reefer containers are built to maintain temperatures, not to lower them. The container itself, however, should not be pre-cooled, so as to minimize interior condensation, which weakens the packing materials and hinders the functioning of the reefer unit. Also, never load cargo above the internal red line.