Skin Cysts

What Causes Them: Often, blocked hair follicles. "Your skin is like a stack of Pringles, with lots of layers. And the top layers usually slough off," says Daniela Kroshinsky, MD, an instructor in dermatology at Massachusetts General Hospital and Harvard Medical School. "But if a hair gets trapped or a piece of skin blocks the follicle, the skin forms a little sac."

How to Get Rid of Them: "Don't squeeze! The cyst could rupture under the skin, leading to inflammation, infection, and scarring," Kroshinsky says. "And since you can't remove the sac, it will just fill up again." Instead, ask a dermatologist to remove it. "Through a small incision, the whole sac is popped out. There's hardly a scar."

Why They Come Back: Blame genes, your age (epidermal cysts are more common in your 30s and 40s), or your acne—it, too, increases your odds. On the other hand, being female lowers your odds: Men get twice as many skin cysts.

Should I Worry?: No, although if you notice a new bump on your skin, it's smart to consult a doctor. "Epidermoid cysts are almost always noncancerous, but don't diagnose yourself," Kroshinsky says. "There's a chance that it's a different type of growth that needs early treatment."