  The spontaneous breaking of symmetries is a widespread phenomenon in physics.
When time translational symmetry is spontaneously broken, an exotic
nonequilibrium state of matter in which the same structures repeat themselves
in time can arise. This state, known as "time crystal", attracted a lot of
interest recently. Another relatively new research area deals with active
matter. Materials consisting of colloidal particles that consume energy from
their environment and propel themselves forward can exhibit intriguing
properties like superfluidity that were previously known only from
quantum-mechanical systems. Here, we bring together these - at first glance
completely different - research fields by showing that self-propelled colloidal
particles can form classical continuous time crystals. We present a state
diagram showing where this new state of matter arises. Furthermore, we
investigate its tempomechanical properties and present a temporal stress-strain
diagram showing parallels to conventional materials science but also remarkable
new properties that do not have a counterpart in common materials.
