46,000 feet.
Pressurized airliner?
Most airliners are pressurized to about 8000MSL equivalent, which is enough for the following experiment:
Open a cheap plastic water bottle (the thinner the better) at altitude and drink its contents. Ensuring that it is in no way crushed while doing so, and still at altitude, cap it tightly. Don’t let the stewardess take it; you’re doing science! Observe what happens to it as you descend. This works best if you’re landing at a near sea level airport.