system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do NASA scientists know that a spacecraft is completely air sealed before it goes into space?
gpt
The same way we test any other pressure sealed system, raging from big tankers to tanks, to the gas pipe systems. Every valve and opening is sealed, then we pressurise the system, if the pressure doesn't drop then it is sealed.  


Once they close the door to the spacecraft, they can pressurise it and check whether the pressure stays stable.