Qualitatively, confinement in dual superconductor models can be understood as a result of the dual to the Meissner effect. The Meissner effect says that a superconducting metal will try to expel magnetic field lines from its interior. If a magnetic field is forced to run through the superconductor, the field lines are compressed in magnetic flux "tubes" known as fluxons. In a dual superconductor the roles of magnetic and electric fields are exchanged and the Meissner effect tries to expel electric field lines. Quarks and antiquarks carry opposite color charges, and for a quark–antiquark pair 'electric' field lines run from the quark to the antiquark. If the quark–antiquark pair are immersed in a dual superconductor, then the electric field lines get compressed to a flux tube. The energy associated to the tube is proportional to its length, and the potential energy of the quark–antiquark is proportional to their separation. The potential energy of colored objects becomes infinite in the limit of large separation, all else being equal, though in reality, when it becomes large enough to form a new quark-anti-quark pair from the vacuum, these split the flux tube and bind to the original anti-quark and quark. A quark–antiquark will therefore always bind regardless of their separation, which explains why no unbound quarks are ever found.