A rare spectroscopy technique directly quantifies the energy required to bind two excitons together. The experiment harnessed interactions between real and virtual states to 'switch' the electronic state of an atomically-thin (2D) material. As well as improving fundamental understanding of biexciton dynamics and exotic new quantum materials, the study aids work towards biexciton-based devices such as compact lasers and chemical-sensors, and the search for future low-energy electronics based on topological materials.