system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why carbonated drinks stay "fresh" for ages until you open it, but after reclosing the lid very tightly it goes flat in a few days

This always frustrates me. I leave a 2 liter of coke in the pantry, its fine for a *month*. I open it, pour a glass, close that **** as tightly as possible and put it back. 3 days later theres next to no fizz. How come?
gpt
The bottles are pressurized to keep the CO2 in the liquid. When you open it, the bottle depressurizes and the CO2 can freely escape into the void you make by drinking some. After the bottle is opened once, you'll hear it depressurize again the second time you open it. This is the gas that has escaped since you last opened it.