Tecmo Super Bowl was a revolutionary game not just because it was the first video game to feature both real NFL rosters and real NFL logos, but also because it did what seemed like a miracle: The game saved your season progress as you played and kept track of your stats, even when the game was turned off. For most people I’ve asked, the answer to how the game did this was a mystery.

Turns out it’s frighteningly simple: Part of the game cartridge never turned off, thanks to a watch battery inside the game.

The guys at eStarland.com, a retro game retailer and repair shop in Chantilly, Virginia, cracked open a copy of Tecmo to show us how it worked. They also told us that the battery doesn’t last forever: It was supposed to last about 20 years. Many cartridges (including mine) have lasted longer, but the store gets people in with dead batteries for cartridges (especially The Legend of Zelda, the first game to feature this technology) about once per week. They showed us how to replace the battery yourself–with a soldering iron–but you can also have a store like theirs do the fix for about $5.