# How do thermoses know if they are supposed to keep drinks hot or cold?
**URL:** https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114
**Category:** Factual Questions
**Created:** [August 23, 2010, 12:05am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114 "2010-08-23T00:05:05Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@PSXer](https://boards.straightdope.com/u/PSXer)
#### Post date: [August 23, 2010, 12:05am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/1 "2010-08-23T00:05:05Z")
my friend wants to know
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### Author:  [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)
#### Post date: [August 23, 2010, 12:06am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/2 "2010-08-23T00:06:06Z")
They don’t. They prevent the radiation or convection of heat, in or out.
🙂
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### Author:  [@running\_coach](https://boards.straightdope.com/u/running_coach)
#### Post date: [August 23, 2010, 12:08am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/3 "2010-08-23T00:08:45Z")
There’s millions of tiny elves that keep the hot molecules and the cold molecules where they belong.
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### Author:  [@JoelUpchurch](https://boards.straightdope.com/u/JoelUpchurch)
#### Post date: [August 23, 2010, 12:13am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/4 "2010-08-23T00:13:42Z")
> [@runner pat](#):
>
> There’s millions of tiny elves that keep the hot molecules and the cold molecules where they belong.
They aren’t elves, they are demons. Maxwell’s demons.
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### Author:  [@running\_coach](https://boards.straightdope.com/u/running_coach)
#### Post date: [August 23, 2010, 12:19am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/5 "2010-08-23T00:19:09Z")
> [@JoelUpchurch](#):
>
> > [@runner pat](#):
> >
> > There’s millions of tiny elves that keep the hot molecules and the cold molecules where they belong.
>
> They aren’t elves, they are demons. Maxwell’s demons.
They have the silver hammers and they cause headaches.
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### Author:  [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)
#### Post date: [August 23, 2010, 12:26am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/6 "2010-08-23T00:26:00Z")
> [@JoelUpchurch](#):
>
> > [@runner pat](#):
> >
> > There’s millions of tiny elves that keep the hot molecules and the cold molecules where they belong.
>
> They aren’t elves, they are demons. Maxwell’s demons.
[Max Planck?](http://en.wikipedia.org/wiki/Planck_constant)
Coffee thermoses work from quantum mechanics?
😕
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### Author:  [@John\_Mace](https://boards.straightdope.com/u/John_Mace)
#### Post date: [August 23, 2010, 12:29am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/7 "2010-08-23T00:29:17Z")
Your friend hasn’t been telling a certain joke, now, has he? One about the greatest invention in the world?
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 23, 2010, 12:33am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/8 "2010-08-23T00:33:19Z")
No, [James Clerk Maxwell](http://en.wikipedia.org/wiki/James_Clerk_Maxwell), probably the third or fourth greatest physicist in history. He’s most famous for completing the classical understanding of electromagnetism, but he also did a lot of important work in thermodynamics.
In all seriousness, all a thermos does is it keeps what’s inside of it from exchanging heat with its environment, and thereby in practice keeps the contents at the same temperature they were at when they were put in. If you boil lemonade and put it in one thermos, and freeze coffee and put it in another, the lemonade will stay hot and the coffee will stay cold, even though they’re “supposed” to be the other way around.
Put it another way: A wall can either keep people inside, or keep them outside. How does it know which to do? It doesn’t; it just keeps people on the same side they start on. A thermos is like a wall, but one that blocks heat, not people.
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### Author:  [@Smeghead](https://boards.straightdope.com/u/Smeghead)
#### Post date: [August 23, 2010, 12:34am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/9 "2010-08-23T00:34:51Z")
Is your friend about to ask us why we park on a driveway and drive on a parkway? That makes me feel something ending in -gry, but not angry or hungry.
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### Author:  [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)
#### Post date: [August 23, 2010, 12:39am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/10 "2010-08-23T00:39:05Z")
They don’t know. A thermos is pretty stupid so you have to tell it what to do with the contents, like, “keep this soup hot thermos.” I take advantage of this by filling mine with water and telling it, “keep this Czech style wheat beer ice cold thermos.” Delicious.
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### Author:  [@cochrane](https://boards.straightdope.com/u/cochrane)
#### Post date: [August 23, 2010, 12:41am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/11 "2010-08-23T00:41:09Z")
It’s like a McDLT box. It keeps the hot side hot and the cold side cold.
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### Author:  [@PSXer](https://boards.straightdope.com/u/PSXer)
#### Post date: [August 23, 2010, 12:44am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/12 "2010-08-23T00:44:22Z")
In the McDLT commercial the cheese is on the cold side. Shouldn’t it be on the hot side so it melts onto the meat?
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### Author:  [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)
#### Post date: [August 23, 2010, 12:45am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/13 "2010-08-23T00:45:28Z")
> [@cochrane](#):
>
> It’s like a McDLT box. It keeps the hot side hot and the cold side cold.
Do they still sell those things?
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### Author:  [@johnpost](https://boards.straightdope.com/u/johnpost)
#### Post date: [August 23, 2010, 12:47am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/14 "2010-08-23T00:47:08Z")
wizard named Dewar did it, caused all thermos bottles to behave like that forever.
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### Author:  [@Animastryfe](https://boards.straightdope.com/u/Animastryfe)
#### Post date: [August 23, 2010, 3:56am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/15 "2010-08-23T03:56:44Z")
> [@Chronos](#):
>
> No, [James Clerk Maxwell](http://en.wikipedia.org/wiki/James_Clerk_Maxwell), probably the third or fourth greatest physicist in history. He’s most famous for completing the classical understanding of electromagnetism, but he also did a lot of important work in thermodynamics.
If you were to consider him the fourth, whom would you consider the third?
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### Author:  [@Jenaroph](https://boards.straightdope.com/u/Jenaroph)
#### Post date: [August 23, 2010, 4:00am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/16 "2010-08-23T04:00:08Z")
> [@PSXer](#):
>
> my friend wants to know
“Tootie.”
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### Author:  [@markdash](https://boards.straightdope.com/u/markdash)
#### Post date: [August 23, 2010, 4:40am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/17 "2010-08-23T04:40:32Z")
> [@don\_t\_ask](#):
>
> They don’t know. A thermos is pretty stupid so you have to tell it what to do with the contents, like, “keep this soup hot thermos.” I take advantage of this by filling mine with water and telling it, “keep this Czech style wheat beer ice cold thermos.” Delicious.
Yeah, one time I totally screwed up and forgot to tell my Thermos to activate hot mode, so when I went to drink my hot chocolate, I ended up with cold chocolate! It ruined my day.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 23, 2010, 4:44am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/18 "2010-08-23T04:44:46Z")
> [@](#):
>
> If you were to consider him the fourth, whom would you consider the third?
Newton and Einstein are the top two, then maybe Archimedes, then Maxwell.
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### Author:  [@Nicolas\_Bourbaki](https://boards.straightdope.com/u/Nicolas_Bourbaki)
#### Post date: [August 23, 2010, 5:09am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/19 "2010-08-23T05:09:49Z")
> [@carnivorousplant](#):
>
> > [@cochrane](#):
> >
> > It’s like a McDLT box. It keeps the hot side hot and the cold side cold.
>
> Do they still sell those things?
In the world **PSXer** and his friend seem to live in? Probably.
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### Author:  [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)
#### Post date: [August 23, 2010, 5:19am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/20 "2010-08-23T05:19:16Z")
> [@PSXer](#):
>
> In the McDLT commercial the cheese is on the cold side. Shouldn’t it be on the hot side so it melts onto the meat?
Yes. That’s why McDLT’s are disgusting.
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