Originally Posted by Flaflyer

I amuse myself and fellow pax by keeping a pint disposable water bottle at my seat. I carry an empty one through TSA and fill it airside or in the plane to have my own supply esp. on long transocean flights.

The Fasten Seatbelt and Turn Of Electronics warnings are at 10K feet. Since the cabin is at about 8K pressure altitude, I finish the water and tightly cap the bottle and place it in my lap or on the tray table. Sure enough right about 8K feet the bottle starts imploding and by about 1K feet is really smashed. Nonscientific seatmates watch the slow motion crush and have to ask “Why is that bottle doing that?” I tell them the bottle is my Poor Man’s Passenger Altitude Indicator.