Push most objects and they will accelerate in the direction they were pushed. Not so for objects with a negative effective mass, which do the exact opposite by accelerating against an applied force. This counter-intuitive behaviour has now been realised experimentally in an expanding Bose–Einstein condensate made from rubidium atoms. Mohammad Khamehchi of Washington State University and colleagues engineered a specific dispersion relation in the system using Raman dressing techniques to open a gap at the crossing of two parabolic free-particle dispersion curves. For suitable parameters this leads to a branch with a region of negative curvature, and thus a negative effective mass. The team reports a range of interesting dynamics in the system, including breaking of parity and Galilean covariance, dynamical instabilities and self-trapping.