Superconductors conduct current without loss, but usually require extreme cold. Hg1223 — a mixture of mercury, barium, calcium, copper and oxygen — breaks the rule: it stops resisting current already at –139°C. The secret is that the strong electrical repulsion between electrons forces them to pair up. It’s like a crowded subway car: everyone tries to keep their distance, but in the crush people grab onto each other to avoid falling — and so electrons form stable pairs.

Pressure helps: electrons start hopping between atoms more often, and repulsion weakens, raising the working temperature to –113°C. Magnetism, usually an enemy of superconductivity, here does not harm but strengthens the pairs. Understanding this leads to the dream of room-temperature superconductors, which would revolutionize energy.