Why Onions Make You Cry?

Your eyes really "water" a little all the time. All of our special sense organs—the tongue, the nose, the ears and eyes are kept moist by special glands. They are exposed to the air and must be kept clean. Moisture probably makes the nerves of the sense organs more sensitive to impressions. The inside of our hands and fingers, where we feel most keenly, are moister than the backs. Now, when there is trouble in a sense organ, like too strong a taste in the mouth, or a "cold" in the nose, or a grain of dust on the eye-ball, the moistening fluid just gushes out to wash it away. Then the eyes or nose or mouth "waters" or overflows.
The eye is irritated by many things beside dust. The fumes of vinegar, pepper and onions make the eyeballs smart. Instantly the tear-glands overflow to protect the eyes. If they could not do this the eyes would become inflamed, and they might be permanently injured. Just watch now and see if onions and pepper do not make you sneeze, and make your nose "water," too, as well as your eyes. Think of lemons and feel your mouth "water."