Relaxation and failure@footnote 1@ of C or BN@footnote 2@ nanotubes can be reversed in a well-defined sequence of atomic rearrangements. This process corresponds to cap-to-cap coalescence or welding of two nanotubes. Precise mechanism is revealed by topological analysis@footnote 3@ and consists exclusively of number of Stone-Wales bond rotations, following the primary jump-to-contact polymerization with covalent bonding. This mechanism explains several "natural" phenomena like diameter doubling of nanotubes, coalescence of C@sub 60@ in peapods, etc. It also generates a series of stable intermediate neck-shaped hetero-junctions with potentially useful electronic properties due to electrostatic dipoles,@footnote 4@ density of states and conductance all varying along the series of emerging structures. @FootnoteText@ @footnote 1@ B. I. Yakobson, Ph. Avouris, Topics Appl. Phys. 80, 287-329 (2001); Ge. G. Samsonidze, Gu. G. Samsonidze, B. I. Yakobson, Phys. Rev. Lett, 88, 065501 (2002).@footnote 2@ H. F. Bettinger, T. Dumitrica, G.E. Scuseria, B. I. Yakobson, Phys. Rev. B, 65, Rap. Comm,, 041406 (2002).@footnote 3@ Y. Zhao, B. I. Yakobson, R. E. Smalley, Phys. Rev. Lett., 88, 185501 (2002).@footnote 4@ T. Dumitrica, C. Landis, B. I. Yakobson, Chem. Phys. Lett. (submitted).