  High energy heavy ion collisions create quark gluon plasmas that behave like
almost perfect fluids. Very similar features to those that led to this insight
have also been observed in experimental data from collisions of small systems,
involving protons or other light nuclei. We describe recent developments aimed
at understanding whether, and if so how, systems that produce relatively few
particles (orders of magnitude less than in typical heavy ion collisions) and
are only one to a few times the size of a proton, can behave like fluids. This
involves a deeper understanding of fluid dynamics and its applicability,
improvements of our understanding of the initial geometry of the collisions by
considering fluctuations of the proton shape, as well as advancements in the
calculation of initial state effects within an effective theory of quantum
chromodynamics, which can affect the observables that are used to study fluid
behavior. We further address open questions and discuss future directions.
