Once a hair comes out of the scalp, it's dead. Delightful thought, no? Even though hair outside of the scalp is no longer living, scientists can still tell a lot about a person just from analyzing his or her strands.
Your hair, which grows about a half-inch per month, comes out from living follicles from within the scalp, so whatever you put in your body becomes pretty much trapped "evidence" within. For an accurate drug analysis, for instance, it's typical for scientists to look at about one and a half inches of hair (about 90 days' worth) towards the newer growth. That's why hardcore drug users often shave their heads — to avoid detection.
Hair analysis can also tell approximately where you live, your exposure to chemicals, your DNA information, if you drink, and your eating habits. One thing hair can't reveal? Surprisingly, it can't tell your gender, according to the FBI — and the g-men should know. Did you ever think that your hair was like a living diary? Turns out, it is.