Imagine you're making someone a tea, but for whatever reason they have to leave soon. How can I cool the tea so that they don't have to choose between leaving it unfinished or burning their mouth? Ideally the method should not only be fast, but also let me reliably reach about the same temperature every time.
I've thought of the following:
Add additional milk - cools tea, but affects taste.
Heat the water less - won't make tea as hot, but might affect how well the tea infuses.
Add cool water to boiled water - would cool down a fully-infused tea, but hard to control resulting temperature.
Put it in the fridge for 30 seconds or so - would cool tea, but seems like a bad idea.
Con: Inexact scienceon the contrary, thermodynamics is a very exact and well understood science. In a nutshell, the final temperature is the weighted mean of the temperatures of both liquids (assuming nothing else influences the temperature, which on very short period is close enough). e.g. to cool 250ml of 70* tea to 60*, simply add 50ml of 10* water to it.