It's perhaps a good job that most astronauts seems to be trained engineers and physicists — because a stay aboard the International Space requires the maintenance of some fairly complex equipment.
While it looks like it could be a car engine, in this image Scott Kelly and Terry Virts performs maintenance on the Carbon Dioxide Removal Assembly that sits in the Japanese Experiment Module. It's a pretty important piece of kit: it scrubs the air aboard the station to remove the carbon dioxide, allowing those aboard to breathe in safety. No pressure, boys. [NASA]