 President Joe Biden tripped and fell after handing out the last diploma at a graduation ceremony at the U.S. Air Force Academy on June 1. The 80-year-old U.S. president quickly got up on one knee, helped by three people and walked back to his seat unassisted. As Biden was helped up, he pointed behind him, seeming to indicate what he tripped over. White House communications director Ben LeBolt said on Twitter that Biden was fine. There was a sandbag on stage while he was shaking hands, he explained. The fall came after Biden delivered a commencement address to a flag-waving audience where he warned graduates they will enter service in an increasingly unstable world, citing challenges from Russia and China.