Chemists have constructed liquid crystals with optical properties that can be instantly and reversibly controlled by an external magnetic field. The research opens the door to display applications relying on the instantaneous and contactless nature of magnetic manipulation -- such as signage, posters, writing tablets, and billboards. Requiring no electrodes, the liquid crystals have applications in anti-counterfeit technology and optical communication devices for controlling the amplitude, phase, polarization, propagation direction of light.