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Garlic is often called "the stinking rose." That's because it contains molecules composed of the same atoms that lurk in burnt matches and rotten eggs. But if you put a whole clove of garlic right up to your nose, you won't smell much. The molecules that create the garlic smell are not actually present in natural garlic. They are synthesized in a reaction that occurs when garlic is cut or crushed. When a knife slices through garlic, cell membranes rupture, releasing an enzyme called allinase. Allinase can chemically change a tiny, odorless molecule called alliin into allicin. Allicin is the pungent, sulfur-containing molecule that can alienate friends who get too close and add zest to bland food. During any chemical reaction, atoms rearrange themselves into new substances. But all chemical reactions do not proceed in the same way. Some transformations occur spontaneously and explosively while others are hard to initiate and proceed as slowly as a rusting fence. The ease and speed with which molecules change or undergo chemical reaction depend on how often they collide and the energy needed to get the reaction started. Molecules are surrounded by negatively charged electrons which repel the electrons in other molecules. For each reaction, molecules must move fast enough to overcome these repulsive forces. This energy barrier is like crossing a mountain. Just as an ocean wave may have enough energy to knock you off your feet, a molecular collision must have enough energy to break chemical bonds. If the reactant molecules have enough energy, they can climb the energy mountain, react, and fall down the other side as products. Usually you can give molecules extra energy and make them collide more often by heating them up. But alliin and allinase only need to meet at room temperature to create allicin. The secret is in the structure of the allinase enzyme. One section of the molecule is called the active site. In a typical enzyme, the active site looks like a dent or crevice in the side of the molecule. The shape of the crevice in allinase exactly matches the molecular shape of alliin. The enzyme and substrate fit together like a key fits a lock. At the active site, alliin is stretched and twisted until chemical bonds holding it together snap and allicin is formed. Allicin, which no longer fits the enzyme, drifts away, leaving allinase unchanged and the whole process starts again. The enzyme facilitates the reaction by reducing the energy needed to break chemical bonds. Alliin and allinase have found a low-energy tunnel through the energy mountain. After ingestion, the odoriferous sulfur molecules circulate in the bloodstream and escape from your body through exhaled air and perspiration--as any nose will tell you. Why don't some people like the smell of garlic? What smells are unpleasant to you? How does garlic get into your breath? What steps can you take to minimize garlic breath?