Last December, a team of scientists made history by creating a fusion reaction that—for the first time ever—gave off more energy that it took to start. It’s a groundbreaking milestone.We talked to two researchers who were part of that team—Sabrina Nagel and Matthias Hohenberger—about what exactly happened, why it’s been decades in the making, and why it’s such a big deal. This is everything you need to know about their team’s fusion breakthrough.