i also heard the trick that if you come inside from cold temperatures you should only wash your hands with cold water until the numb sensation goes away. why is this?
In: Biology
Your body doesn’t actual sense temperature but differences in temperature. Well, more specifically it sense the rate of heat transfer. And that is directly proportional to differences in temperature. The higher the difference in temp the higher the rate of heat transfer.
So if you hands are cold and you use hot water that’s a big temperature difference. So you perceive it as extra hot.
Another element is that if you have hypothermia, warning the extremities first—rather than the core—[can cause shock and possibly heart failure](https://health.mo.gov/living/healthcondiseases/hypothermia/index.php#:~:text=Warm%20the%20body%20core%20first,warm%20the%20victim%20too%20fast).
It’s OK to use out-of-the-tap hot water. The pain you feel is just your body re-calibrating. Where people get in trouble is that they use water that’s hotter than hot tap water, and they ignore the pain because they figure it’s just the re-calibrating. But you can damage yourself **really** badly, just like how you could damage yourself on a normal day by pouring scalding-hot water on yourself.
A lot of the other comments are half right. Hot water on extremely cold hands can cause tissue damage. You need to warm your hands gradually to avoid that, which is why you use cold/room temperature water.
If you’re numb, then you can’t really tell how hot the water is, and could easily cause burns before you even realize it.
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