Here’s how airplane doors work, and why you shouldn’t open them, according to AsapSCIENCE. The higher a plan goes, the less air pressure there is on the door, although it remains constant throughout a flight—the exertion of an object about 10,000 feet above sea level, even if the plane is going to 40,000 feet. If you got that door opened, it would de-pressurize in half a second. Anything not bolted into the plane itself—drinks, peanuts, the in-flight magazine, passengers walking to the bathroom—would get sucked out immediately, what with the sudden and bizarre discrepancy in air pressure.
Much of the breathable oxygen would also disappear, and if you didn’t get one of those drop-down masks on in time, hypoxia would set in. That’s a critical lack of oxygen, which causes nausea, dizziness, unconsciousness, and death. Then the pilot would have to restore order by balancing pressure, taking the plane down to 10,000 immediately.
So that’s it. So go forth with that knowledge and don’t open the door on your next flight, okay?