High-density polyethylene is technically any polyethylene with a density of 0.93 to 0.97 g/cm³. The density in polyethylene is controlled mostly by the number of branch points along the polymer chain. HDPE has very few branches, so the chains can stack together very closely. This tight packing makes it a very strong polymer, so it’s used to make things like bottlecaps, milk jugs, and Hula Hoops.