Which are the most common everyday phenomena that we don’t properly understand?

Off the top of my head: • Lightning (how does it happen?) • Sleep; dreams (why do they exist?) • Glass (thermodynamics of formation) • Turbulence (when does it start?) • Morphogenesis (how does a creature know what should go where?) • Rain (it seems to start faster than models would predict) • Ice (dynamics of slipperiness) • Static electricity (which material will donate electrons?) • General anaesthetic. (And the mechanism of a lot of drugs, e.g. paracetamol.) That is from Patrick Collison. It...