A deepfake is a video created with artificial intelligence to make a person appear to say or do something they didn't actually do.
In a 17-second clip that the Democrats published on Twitter, Biden can be heard talking about the storming of the Capitol.
One user created a mash-up of this clip and another clip purportedly from the same day, pointing out that the differences in Biden's appearance were too great for it to be "one and the same Joe Biden."
This has led to countless theories in the thread.
And the most common: The Democrats' video is a deepfake.
"He doesn't blink"
Something that many people point out is that Biden does not blink once in the 17-second clip, which has over 8.6 million views on the Democrats' official Twitter account at the time of writing.
- A person blinks on average around 20 times per minute but this can vary, says Rune Brautaset, professor of optometry, to SVT Nyheter.
The number of blinks is thus not enough to determine whether the clip is fake or not, and deepfakes can also blink.
The White House has also posted the entire clip on YouTube where Biden speaks, which is 11 minutes long, where you can see the president moving his eyelids.
In addition, today's deepfake technology is so advanced that it can simulate lifelike movements such as blinking and wrinkling.
That's what Michael Felsberg, professor of computer vision at Linköping University, says.
- There is no reason to "fail" on that front, if you say so, says Professor Michael Felsberg.
Watch as Michael Felsberg, professor of computer vision, goes over the details of the viral clip.