They fluoresce! When light is not being a wave, it can be thought of as packets of energy called photons. The colour of the light decides what energy your photon is. When you shine a laser at one of these particles, it absorbs a photon. This excites an electron, which means that the electron takes in the energy of the photon making it jump up to higher state. But electrons prefer to be in the lowest state possible, so it loses a bit of energy by relaxing in the high state, then it jumps back down to the original low energy state. And to do this it releases another photon. But because it lost some energy relaxing, the new photon has less energy. This means that the new photon is a different colour. So, if you shine a blue laser onto my nanoparticles, you get red light back. This is called fluorescence! It can be used as a harmless way of marking molecules to study them or can be used as an imaging tool called Fluorescence Microscopy.