What Is a Time Crystal?

A time crystal is a phase of matter that spontaneously breaks this translation symmetry in time to show you some kind of periodic pulsing forever. So it's like the emergence of a clock in a system. But importantly, this should happen spontaneously, which means without any feed of energy or without any drain of energy. Okay, because, you know, battery operated clocks are all around us. You can buy one on Amazon, right? Ah, okay, good. All right, so now that we understand better what a crystal is, then what's a time crystal?