The universe today is sparse and cold. However, going backwards in time, the universe gets denser and hotter. At some point, mere microseconds after the big bang, the universe is so dense and so hot that ordinary nuclear matter melts into a plasma of its constituent quarks and gluons. This phase of matter is called the quark-gluon plasma (QGP), and the goal of heavy-ion physics is to recreate this phase of matter in the laboratory using collisions of two large nuclei at relativistic speeds. Recent experimental results suggest that not only have we succeeded in creating the QGP in the lab, but that QGP properties appear even in collision systems previously thought to be too small for QGP formation, like proton-nucleus collisions. In this colloquium, we discuss the latest results from these "small systems" and their implications, including searches for QGP properties in leptonic and semi-leptonic collisions.
