Im not sure its a myth but my girlfriend claims that if u leave the cap of a coke bottle, that it looses less gas instead off closing it and opening it again when u wanted to drink.
She sais that the opening of the bottle looses much more gas cause u can hear the "pssss" every time u open it.

This has caused allot of discusions in our relation, u cant believe how manny times a full open bottle got knockt over on the floor here.

I wanne clear things out, and want to be abble to tell here that closing a coke bottle is preserves the gas more than letting it open.

Srr for the writing mistakes but im from belgium and i talk better english than writing.

Greeting from belgium.

The hiss that you get when reopening indicates that there is still some fizz in there.
If you look at the opened bottle, you will notice bubbles rising. These are dissolved gas escaping to atmosphere. The resealed bottle will have fewer bubbles, and when the pressure rises enough, they will stop. The hiss is this pressure being released.
Of course, if her technique for opening the bottle involves shaking it about before removing the cap, she will be right.