Put a rubber band on the table and pull on both ends: it gets longer. Something much the same happens to certain crystals when you place them in a strong magnetic field. The catch is that crystals usually change their length by a negligible amount, one you would never see without precision instruments. But crystals made of holmium, iron and oxygen stretched by 60 percent under the field, and dysprosium ones by a full 120. It is as if a rubber band doubled in length simply because you brought a magnet near it.

Why does this happen? A crystal contains iron atoms and atoms of a rare-earth metal: holmium or dysprosium. These rare-earth atoms have a special property: their magnetic arrows are very strongly tied to their position in the crystal. When an external magnetic field rotates those arrows, the atoms shift, and the whole crystal stretches like the bellows of an accordion. What is more, it stretches in only one direction, while in the other two it barely changes. The discovery came from comparing three similar crystals: in one of them, the gadolinium one, the magnetic arrows are only weakly tied to position, and it hardly stretched at all. So the whole effect comes down to rare-earth atoms with a strong coupling.

What is it good for? Such crystals could become the basis for devices that change shape on command from a magnet: tiny valves, switches or sensors, for instance. True, for now all of this works only at very low temperatures, close to absolute zero. But the principle has been found, and scientists will now be looking for materials that behave the same way at ordinary temperatures.