An aphid is a tiny, soft insect that lives on plants and drinks the juice from young stems and leaves. Gardeners know them as a pest: a crowd of them can weaken a plant, curl the leaves, and spread plant disease. Many of the common ones look green, though some are black or other colours, and they cluster on new growth rather than flying around your face. British English often calls the green ones greenfly, but they are not flies in the everyday sense. Though actually, ants sometimes "farm" aphids for the sweet liquid they give off, so you may see ants walking among them as if they were keeping livestock. Aphids do not bite people; they are built for sucking plants.