When a beam from a laser or some other coherent light source scatters off a random distribution of matter, it produces an interference pattern of light and dark “peckles” that is uniquely determined by the instantaneous matter distribution at the scale of the light's wavelength. And if changes occur in the sample as a function of time, the speckle pattern evolves to reflect those changes. Physicists exploit this phenomenon when they investigate the diffusion of micron‐sized particles in liquids, for example, with techniques like photoncorrelation spectroscopy and dynamical light scattering.