In 2013, Yitang Zhang, a little-known mathematician who was living in New Hampshire, solved a century-old problem. As Alec Wilkinson reports in this week’s issue of the magazine, Zhang's landmark proof is “a back-door attack on the twin-prime conjecture,” which states that there are an infinite number of prime-number pairs separated by two.

In this video, Wilkinson explains in layman’s terms how Zhang reconceived a mathematical problem that once seemed hopeless.