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Condensed matter: Magnetic collectivity A signature of an exotic state of matter is identified in a magnetic material by researchers in China and Germany. The results from Huiqiu Yuan from Zhejiang University and co-workers indicate a new class of topological materials. In the right circumstances, the electrons in a material work collectively to behave like particles known as Weyl fermions. Weyl fermions have effectively no mass, making them a fascinating test-bed for novel physics. Yuan and colleagues observed a pronounced negative magnetoresistance when a magnetic field was applied in a direction parallel to an electric current passing through caesium antimonide at low temperature. This signature of a Weyl-fermion state was further supported by electronic-structure calculations. While these evasive particles have been identified in a number of materials in the past, caesium antimonide is different because it is ferromagnetic.