Constraining symmetry Most superconductors have only one transition point, and below a certain temperature, their electrical resistance drops to zero. In very rare cases, another superconducting transition appears at a lower temperature. By measuring its specific heat, Hayes et al . reveal that this two-step superconductivity occurs in the compound uranium ditelluride. Complementary optical measurements indicated the breaking of time reversal symmetry, constraining the possible symmetries of the order parameter in this material. —JS