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30+ years ago my mother was pouring hot tea from one cup to another and back presumably to make it cooler. Now, my wife does the same for our kids. And I wonder, what is the physical mechanism that justify this?

I've had my last physics classes about 15-20 years ago and I wasn't the best student of this, but from what I remembered is that adding energy should actually increase, not decrease temperature.

I perform mechanical manoeuvres (a force that is required to literally transfer the fluid from one pot to another) in this process. Plus, another energy is added coming from the fact that moving (in the air) fluid must overcome resistance of the air particles in this way. Shouldn't it all actually increase the temperature of the fluid itself?

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  • $\begingroup$ Clearly pouring hot tea into a room temperature cup results in some heat transfer from the tea to the cup reducing the temperature of the tea. So I don’t understand why you are confused $\endgroup$ – Bob D Nov 24 '19 at 12:30
  • $\begingroup$ The second (B) cup has room temperature only before first pouring. After first "round" (i.e. A -> B -> A), the second cup's temperature is very little less than tea's temperature. So, I am generally confused, if the whole idea isn't pointless. $\endgroup$ – trejder Nov 25 '19 at 13:31
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In addition of the tea giving heat to the colder cup, when you pour from cup A to cup B. Cup A is empty for a short while and give off a little heat to the air. When the tea is returned to A, A is slightly colder. An interesting question would be what's the optimal time of leaving the tea in the colder cup before pouring to the other.

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  • $\begingroup$ Don't forget about evaporative cooling. Pouring exposes a greater surface area. $\endgroup$ – PM 2Ring Nov 24 '19 at 13:52

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