A herbicide is a substance used to kill plants you do not want — weeds in a rice field, grass in the cracks of a path, plants that steal water from a crop. Farmers spray it; some gardeners keep a bottle for the driveway. The second half, -cide, means “killer,” the same idea as in pesticide. It is a practical word from farming, news, and science class, not from cooking. Though actually the tidy word hides a range: some herbicides kill almost every green thing they touch; others are made to kill only certain kinds of plants and leave the crop standing. People also stretch it, fairly, to mean “anything used to kill plants,” even a strong salt water or vinegar mix at home. The school picture is usually a chemical in a spray tank.