The acorn is the nut of the oak (genus Quercus) — a single hard-shelled seed cradled in a woody, scaly cup. Bitter and astringent with tannins when raw, it becomes a sweet, starchy staple once those tannins are leached out, and across Korea, the Americas, Iberia and the Mediterranean it has long been ground into flour and pressed into jellies, porridges and breads.