Stress fractures are one of the most common injures in ballet dancers. Keep your bones strong—don’t smoke. A new study published in the Journal of Adolescent Health reports that even low or irregular amounts of smoking during girls' teenage years lowers their bone density. Adolescence is a critical time for bone growth: As much as 50 percent of your lifetime bone accrual happens during these years. Indulging in a cigarette habit—or even just lighting up from time to time—puts your career at risk.