(a) A time crystal has periodic structures both in space and time. Particles arranged in a periodic pattern in space rotate in one direction even at the lowest energy state, determining periodicity in time. (b) An experimental realization of a time crystal proposed by Li et al. uses ultracold ions confined in a ring-shaped trapping potential. The ions form a periodic structure in space and, under a weak magnetic field, they move along the ring, creating a time crystal.