Any object that an electrical current can pass through is called a conductor. All regular conductors like copper wire exhibit resistance, or the loss of electrical current as the electrons pass through the conductor. The energy lost is given off as heat. The resistance of a conductor decreases as the temperature is decreased. But even approaching absolute zero (the lowest possible temperature where all molecular motion ceases), a regular conductor still has some resistance to an electrical current. A superconductor is a material that experiences no resistance to the electrical current passing through it. So an electrical current can be maintained indefinitely in a superconductor with no loss or degradation of energy. To do this, a superconductor must be cooled with cryogenic fluids to reach its superconducting phase. The reasons for a material achieving this phase are complicated. It has to do with the Meissner effect, where the superconductor rejects all external magnetic fields. It also has to do with a concept called Cooper pairs. This is a quantum mechanical phenomenon where electrons in superconductors behave in attracted pairs called Cooper pairs. This pairing of electrons is a result of phonons, or an excited state of vibrational modes between groups of particles. Both of these phenomena contribute to the superconducting phase.