system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some water bottles 'hiss' when you open them similar to soda, even though they arent carbonated?
gpt
Some are still pressurized with a neutral gas, like nitrogen, to help preserve freshness. Others may have simply pressurized from the air and water warming, causing the air to expand and the water to let off some water vapor which expands as well. This creates pressure, and so, the hiss.