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I was asked this question by my 5year old son. I do not know how to explain to him .

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    $\begingroup$ Cold decreases nerve ending firing. It's like a burn; ice feels good (but is not recommended!); warm water would make it hurt more. $\endgroup$ – anongoodnurse Mar 12 '17 at 19:47
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    $\begingroup$ The nociceptors i.e. the nerve cells that relay pain due to injury (mechanical), noxious heat and noxious chemical burns, get sensitized upon stimulus. I guess the injury sensitizes the nociceptors and hence they also become sensitive to heat (I am not finding a valid reference for this, at the moment). What @anongoodnurse says also makes sense but I am not sure if that is true. Extreme cold also elicits painful stimulus. Have a look at this post too. $\endgroup$ – WYSIWYG Mar 14 '17 at 8:32
  • $\begingroup$ I also suggest that you add some more content in the question body. The body does not convey any question. The title should be a one line summary of the body. If you do not have a lot of details to add then you can still restate the title content a bit more elaborately. We also appreciate some basic attempt from the side of the asked. $\endgroup$ – WYSIWYG Mar 14 '17 at 10:16
  • $\begingroup$ as objects heat up they vibrate more at the molecular level which accentuates interactions ... in your case raw nerve endings receive greater agitation ... cold reduces the frequency of this vibratory movement $\endgroup$ – Scott Stensland Sep 14 '17 at 23:59
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2 reasons. The first (as anongoodnurse mentioned) is that cold temperatures temporarily knock out dermal surface nerves (in the winter you can verify that your face goes numb after a while outside).

The other reason is that a knee scrape produces inflammation (swelling and increased temperature). Cold water (or anything cold) counters these: stopping the inflammation temperature increase, then freezing out enzymes and proteins needed for inflammation and causing them to slow down or malfunction.

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