Why do superconductors let electricity flow with no resistance?

Ordinary electrical resistance comes from electrons colliding with atoms and losing energy as heat. In a superconducting state, electrons form Cooper pairs and behave coherently, allowing current to flow without dissipation. The effect was discovered in 1911 by Heike Kamerlingh Onnes, and the best-known theory explaining conventional superconductivity is the BCS theory from 1957. Superconductivity also creates magnetic levitation through the Meissner effect, which expels magnetic fields from the material. Most superconductors require extremely low temperatures, though some modern materials work at higher temperatures under high pressure.