Eye Contact.

What do Marilyn Manson, Star Wars' Darth Maul, and many teens have in common? Weird eyes--or costume (appearance altering) contact lenses, to be exact. The lenses can turn baby blues into wild cat eyes or those of an alien. But since many teens snare and mix the lenses with pals, experts warn they can make you see red, as in eye infections!

"You can trade conjunctivitis (eye inflammation commonly known as "pink eye") back and forth," says Anne Sumers, ophthalmologist (eye M.D.) and spokesperson for the American Academy of Ophthalmology. Because contact lenses become wet and warm on eye contact, "germs and bacteria happily breed there," says Sumers. "It's a matter of hygiene. You don't swap toothbrushes or share underwear with friends."

Another cause for concern: Doctors fit lenses specifically to each person's eyes. If contacts are too loose, you feel the discomfort of thin plastic floating in your eye. Worse, you can scratch your cornea (the thin, clear top eye layer) and infect it. If lenses are too tight, the cornea doesn't get enough oxygen (most contact lenses are made able plastic). Correct ensures that eyes get the necessary amount of oxygen, and correct fit is the best reason for not pooling wacky lenses. Besides, with a price tag higher than the average teen's allowance (about $99 a set, plus eye exams), they may be too much of an eyeful!