Mallory, Malia and Mia Casper, whose sitting-to-standing challenge took off online, are here. Sports medicine and team physician for the WNBA's New York Liberty, Dr. Ashley V. Austin, reveals what these challenges do and don't prove! Neurologist, professor and bestselling author of "The Invincible Brain," Dr. Majid Fotuhi, tries two challenges and separates fact from fiction. And tonal coach and mobility expert Ackeem Emmons demonstrates a workout to help increase mobility.