Recently, Dr. Curtis Cooper (well, his computer) discovered the largest prime number known to humanity. It's more than 22 million digits long, so folks just call it "M74207281" for short.
Okay, so how did Cooper do it? And why? In the video below, mathematician (and standup comedian) Matt Parker digs in. He shows how you can test the primality of a given number, plus various shortcuts that allow computers to work on absurdly large numbers like M74207281.
For more on this topic, Parker interviewed Cooper and posted the result (in weird 50fps high-frame-rate video) to YouTube. And if you want to start looking for prime numbers yourself, check out GIMPS!