  The friction between the walls of multi-wall carbon nanotubes is shown to be
extremely low in general, with important details related to the specific choice
of the walls. This is governed by a simple expression revealing that the
phenomenon is a profound consequence of the specific symmetry breaking:
super-slippery sliding of the incommensurate walls is a Goldstone mode. Three
universal principles of tribology, offering a recipe for the lubricant
selection are emphasized.
