Supersymmetry is a conjectured symmetry of space and time — and a unique one. It has been a very popular idea among theoretical physicists, for a number of reasons, for several decades — it was a hit back when I was a student, before physics was cool, and even well before. An automatic consequence of having this symmetry in nature is that every type of particle has one or more superpartners — other types of particles that share many of the same properties, but differ in a crucial way. If a particle is a fermion, its super-partner is a boson. If a particle is a boson, its super-partner is a fermion. It is a symmetry that relates space and time themselves to superpartner directions of space and time — in other words, space-time itself has extra dimensions quite unlike the ones we know.