When botanists use the term herbaceous, they mean a plant that is soft stemmed, with little woody tissue. But in culinary and other ethnobotanical uses, herbs are usually defined as plants of temperate climates whose leaves are harvested for use. It can get confusing: Although most are nonwoody, some woody plants like rosemary, whose leaves we use, are herbs in a culinary sense.
Invite students to rub the leaves of aromatic herbs to break microscopic oil glands and release their fragrance. Then ask students to imagine why these plants might have evolved with these distinct aromas and flavors. (Botanists believe that they are largely a defense against being eaten by herbivores.) Ask students to brainstorm common herbs in which we eat both the leaves and the seeds (e.g., dill).
Spices, on the other hand, are mainly tropical plants in which we typically use the roots (ginger), fruits (vanilla pods), flowers (cloves), seeds (pepper), or bark (cinnamon).
Consider having students identify the tropics of Cancer and Capricorn, which roughly separate tropical and temperate zones, and compare climate factors in both zones. Invite them to brainstorm which popular flavoring is neither an herb nor a spice. (Salt is a mineral not derived from a plant!)