Thermalization of the world's smallest fluids: Recent developments

The late Gerry Brown was not shy to tackle complex scientific problems that took time to play out but yielded in the end a deeper understanding of many-body phenomena. In this note, prepared for a memorial volume in his honor, we provide a perspective on a couple of outstanding scientific puzzles that have their origin in our understanding of the thermalization of matter in ultrarelativistic heavy ion collisions, and possibly, in high multiplicity proton–proton and proton–nucleus collisions.