Gravity. More than the name of a killer movie, it’s likely something we take for granted every single day. After all, nearly everything we do is reliant on the idea that stuff stays in place when we stop holding it. Astronauts don’t have that luxury, however, and when even simple tasks take a ton of effort, something relatively complex like using a 3D printer is even harder. Why would astronauts need one of those? Well, because stuff breaks in space, and replacing a busted part isn’t as simple as hitting Home Depot.