system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do butter bells work?

So I understand that the point of a butter bell is that your cup of butter is not exposed to the outside air because the water creates a seal. What I don’t understand is that there is still some air trapped between the butter and this water seal because the water doesn’t touch the butter. Is the reason why butter lasts longer because the amount of air and consequently bacteria the butter is exposed to less than if it was not in a seal? I would’ve thought that because the butter has the same surface area exposed to the air with or without the seal, the seal is unnecessary.

Please help, my girlfriend and I are fighting.
gpt
It's not bacteria, it's oxygen. Butter does not go rancid because of microorganisms, it goes rancid because fat naturally degrades over time. Exposure to light, heat, and oxygen makes fat degrade faster. So, even though the butter in butterbell might have some of its surface touching air in a bubble, that air is trapped and not getting new oxygen from the rest of the atmosphere like the air touching the surface of butter in a normal dish.