system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I just bought a thermos that claims it keeps things hot 7 hours and cold 18 hours. Why aren't these numbers equal?
gpt
Hot drinks are hotter than cold drinks are cold.

Cold drinks are rarely colder than freezing, and are still refreshing in the 40^o - 50^o range.  Also, if something is frozen solid, it takes extra energy to thaw it, which means extra time.

You drink coffee at between 120^o and 140^(o), below that it starts getting lukewarm and bitter.  

That means a cold drink is ~20-35^o below room temperature, while a hot drink is ~50-70^o above.  Roughly twice the difference, roughly twice the time.