A charged particle is placed next to a current-carrying wire. The wire produces a magnetic field, but if the particle is at rest, the field exerts no force on it. However, in a different inertial reference frame moving at speed v parallel to the wire, the particle is seen to be in motion, and so there is a magnetic force perpendicular to the wire! Electrostatic force is the same in both cases. Where does the extra magnetic force come from?