Call a tomato an heirloom, and folks may think it means local, organic, juicy, and delicious. While an heirloom tomato can indeed be all of those things, it must meet only one requirement to be called heirloom: Its propagation relies on natural, or open, pollination, such as from insects or the wind. Why does this matter? Most supermarket tomatoes are hybrids, hand-pollinated to ensure certain traits, such as thick skins that help the tomato resist pests or diseases and allow the fruit to be harvested whole by machine and shipped long distances intact. By contrast, heirlooms may be irregular in shape and color and tend to have thin skins that require hand-picking and careful shipping. Though many heirloom tomatoes are grown organically, that is not part of the definition of heirloom and so not a guarantee. Neither is delicious flavor, though heirlooms usually have more locules-the cavities with seeds-than commercial hybrids, and locules are full of flavorful compounds. Plus, since that thin skin means they’re not good shippers, heirlooms tend to be grown locally and often allowed to ripen on the vine, which boosts flavor as well.