Ice cream headache is a familiar summertime sensation, but the pain's source has been mysterious until now. A team led by Jorge Serrador of Harvard Medical School produced brain scans of “second-by-second changes” in blood flow while subjects sipped iced water through a straw pressed against the roof of the mouth, which caused the brain's major artery to widen. “Blood flow changes actually preceded the pain” that subjects reported, Serrador says. As the vessel narrowed again, the discomfort ebbed. He suspects that the influx of blood is meant to protect the brain from extreme cold and that increased pressure inside the skull could cause the pain. Serrador presented the results at Experimental Biology 2012 in April in San Diego.