You’re lying in bed, binging on 90s TV, and suddenly it feels like you turned on an electric blanket. But you don’t have one—this isn’t your grandparents house, after all. All that warmth is coming from the little box in the middle of your laptop charger. As this video from Vox explains, the box converts the alternating current electricity that comes out of the wall socket to direct current, which your laptop runs on. The heat is just the wasted energy from the conversion process. And it’s usually no big deal, unless touching the box makes you want to play hot potato. If it feels more like a stovetop than a charger, something is amiss, and you’ll want to get it checked out.