Research groups in Europe and China have announced the first functioning nuclear clocks. Whereas atomic clocks keep time using an atom's electrons, nuclear clocks rely on the atomic nucleus itself. They are built around the isotope thorium-229, the nucleus of which an ultraviolet laser can push into an excited state. Chinese researchers placed thorium-229 nuclei inside calcium fluoride crystals to build small, portable clocks. Teams from Austria and Germany used a thorium-229 clock to hunt for dark matter but detected no signal. In time, nuclear clocks may beat atomic clocks on precision and help reveal shifts in fundamental forces.