Sanders was filmed clapping very slowly with pursed lips and an unfocused expression during the address to Congress, particularly as the president spoke about his efforts to lift Americans out of poverty, prompting Twitter users to speculate what might be going through his mind.
People suggested Bernie was "contemplating his life right now," that he "just realised he left his oven on at home," he was "thinking about all the things I need to do tomorrow" and that he was clapping "like a man who just watched the love of his life walk down the aisle with the dude that cheated on her 6 times in college".
Sanders confirmed after the speech that he was indeed less than impressed, pointing out that Trump did not mention climate change or college debt once and criticising his plan to privatise US infrastructure, clamp down on legal immigration including the Dreamers, among other policies.
He tweeted: "The good news is that the vast majority of our people have a very different vision for the future of our country than what Trump and the Republican leadership are giving us. In an unprecedented way, we are witnessing a revitalization of American democracy with more and more people standing up and fighting back."
Sanders added: "When ordinary people stand up and fight for justice there is nothing that we cannot accomplish. That has been the history of America, and that is our future."
Sanders was not the only member of the audience with a meme-worthy face. Nancy Pelosi, Minority Leader of the United States House of Representatives, was described as a "sour puss" with "lemons in her mouth" on Twitter after the way she was caught on camera. Country musician Charlie Daniels said: "Last night Nancy Pelosi looked as if she was sitting in a dentists office waiting to get a root canal done."
Reacting to the address, she said today: "While our expectations for greatness and vision from the president are not high, he stooped to a new low."
Democrat Senator Cory Booker was also caught grimacing during Trump's address, and his expression was described as an "all-time fourth-wall breaking classic". He later said the speech was "total propaganda and bluster".