Hi FlyerTalk!

1. I've noticed over the years that when I carry bottles of contact fluid onto a plane, I hear a "whooshing" sound of gas if I open them onboard, or sometimes if I open them at my destination after landing. But it happens so fast that I can't tell whether gas goes in or out. I suspect it has to do with the cabin pressure being lower than atmospheric pressure at sea level..but can someone who knows more about science than I do tell me what's actually going on?

2. Similarly, when I carry bottled mouthwash in my checked bags, I find that it often overflows, even if the cap is closed tightly, if (and only if) the tamper-proof seal has been opened. For this reason, I pack the bottle in a ziploc bag, but it still spills within the bag (and the bag just keeps the spill from getting out). Can anyone tell me...if I want to reduce or prevent the spillage, should I squeeze the excess air out of the bottle before capping and packing it? Or should I let as much air into the bottle as possible before capping and packing it? Or something else?

Thanks!