Mixing mild and cold water, which one to pour first? Suppose for example that a person like me likes his water in-between. A bit colder than the room temperature but not very cold. If you have a water dispenser that pours rtp water and cold water, which one to pour in the cup first to ensure the temperature I'm looking for? 
I was just thinking of it as I was pouring water today.  Could someone provide a general abstract answer, and also a scientific detailed one?
 A: I think that it makes little difference... but anyway...
You should pour the room-temperature water first, and then the cold water until you get the target temperature.
Why? Because if you pour the cold water first, it will immediately start to warm. If you leave it enough time it will actually reach equilibrium at room temperature. So the amount of cold water required will change constantly, depending on the time it takes to pour it.
In contrast, if you pour the RT water first, then its temperature will remain stable until you are able to calculate how much cold water you need. Then you just need to pour that much water and drink it quickly.
A: This is assuming no stirring, since that is a hassle.
There is a third possibility of pouring both in at the same time. That would ensure the fastest convection.
Cold then warm will be slowest since warm water rises.
Warm then cold will be slightly faster since it promotes convection.
This all depends on your personal preferences.
The fastest route to the correct temperature should be pouring both at the same time. You can guesstimate the end temperature by volumes. If they start at the same volume, then they will meet at the middle. The fast convection rate will ignore most of the effects of the room temperature at first.
Depending on which you hate more, I would stop at more favorable of colder or hotter than the goal. Test and add more water.
Edit: If this is for drinking and you don't mind a roundabout way and some effort.


*

*Refrigerate a pitcher of drinking water so you know its temperature.

*Find out the temperature of the hot water.

*Figure out the volumes necessary to reach your desired temperature.

*Etch/mark the level into a drinking cup for the waters. Remember the order.

*Pour separately and stir.

