"I haven't washed my hair with anything but water or dry shampoo in over four weeks," my friend told me via email. My gut reaction was horror, quickly followed by curiosity. Did it smell? Is it good for your hair? And why?! Turns out, my friend is not alone — this hair trend known as no poo (get it?) has been gaining momentum and countless fans. Devotees believe they're doing something good for their strands, favoring more-natural products (such as apple cider vinegar and baking soda) as opposed to traditional, chemical-laden products. While this seems to be logical, I had to question: is shampoo — something almost everyone uses — really that problematic for your hair? I talked to two experts — dermatologist Dr. Dennis Gross and trichologist Elizabeth Cunnane Phillips — to get to the, ahem, root of the issue.