Matthew:

Why do some people have a white moon on their fingernails and others don't? Does this mean you're lacking something in your diet or that you might have a health problem?

-- Moonless, Ramona

The little moons rising over some people's cuticles are called lunulae and are as devoid of meaning and purpose as any body part can be. Their presence or absence is no indicator of physical health., nutritional habits, moral rectitude, personal hygiene, manual dexterity, intelligence, breeding, sexual orientation, bank balance. It's one of the few physical attributes that, no matter how one might try, cannot be used to further separate us human beans into us-and-them categories. The white arcs are merely air trapped between fingernail and nail bed. Some people's nails fit more closely to the skin below and have tiny moons or no moons at all. It's possible to have moons on some fingers and be moonless on others.

There's one esoteric contribution made by lunulae and nail beds. You can check blood circulation in a person's extremities by checking skin color that shows through his fingernails. Well-oxygenated blood reaching the tiny capillaries in your fingers will make the nail beds appear pink; poor circulation will give them a blue cast. But of course this works whether or not you have moons over your manicure.