Ripening in fruits such as a tomato, allows them to taste better because the more ripe, the more sweet it will taste. Ripening in nature involves a gaseous plant hormone called ethylene. This chemical triggers the ripening process. Subjecting plants to wounding and high temperatures also triggers ethylene production. When ethylene is present, fruit ripening genes are triggered. These allow other genes to make pectinases to break down cell walls to soften fruit, amylases to produce more simple sugars from starch and hydrolases to degrade chlorophyll present in a green fruit. This results in a color change associated with ripened fruit. Sometimes, ripening is artificially triggered by gassing fruit with ethylene, using calcium carbide as a ripening agent, or by placing fruit in a bag, to trap ethylene to quicken the ripening process.