A man sunbathes in Malmo, Sweden, in July. Humans (and other animals) make vitamin D when exposed to sunlight. Johan Nilsson/AFP via Getty Images hide caption

itoggle caption Johan Nilsson/AFP via Getty Images

A CT scan of the brain shows the cerebellum, a small portion of each temporal lobe, and the sinuses. Andrew Ciscel via Wikimedia Commons hide caption

itoggle caption Andrew Ciscel via Wikimedia Commons

Megan Lindsey (right) and her friend Alexandria Bodfish at soccer camp at University of Notre Dame.  Megan, 14, suffered concussions twice this fall while playing soccer. Courtesy of Barbara Wirtz hide caption

itoggle caption Courtesy of Barbara Wirtz

Cycling has at least tripled over the past two decades in several big cities across the U.S., including Minneapolis, Chicago and San Francisco. Jonathan Steinberg hide caption

itoggle caption Jonathan Steinberg