The kumquat (Citrus japonica) is a thumb-tip-sized citrus eaten whole, skin and all — and uniquely among citrus, that thin rind is sweet and perfumed while the few segments of flesh inside are sharply sour, so a single fruit delivers a sweet-then- tart burst in one bite. Borne on small, frost-hardy evergreen shrubs native to southern China, the bright-orange ovals are eaten fresh, candied, simmered into marmalade and dropped whole into cocktails.