The friction between two diamond surfaces placed in sliding contact has been investigated using molecular dynamics simulations. These simulations show that the friction between two hydrogen-terminated diamond (111) surfaces is significantly reduced when methane molecules are placed between the diamond surfaces compared to the same two surfaces in the absence of the debris, or third-body, molecules. Macroscopic experiments that have examined the friction of diamond on diamond in the presence of debris (Hayward and Field, Proc. Inst. Mech. Eng., IMechE Conf : 1 (1987) 205) show similar trends. The specific atomic-scale motions which lead to the reduced friction and to the deformation of the methane molecules are identified.