Onions are both aromatics and vegetables in their own right, and they have been cultivated by humans since at least 5000 B.C. Besides cooking, onions have been used for paying rent, increasing fertility in both humans and animals, killing insects, curing baldness and treating snakebites.
Onions are healthful to eat, containing high doses of flavonoids and polyphenols. Some experts suggest eating half an onion every day.
Dogs and cats, however, should never eat onions. They contain sulfoxides, which are toxic to many animals; thus, they should not be fed to pets or livestock in general. The substances that cause eyes to sting and tear when an onion is cut are called sulfenic acids.