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Home experiment time! Go run some hot water, either in a sink or tub, and stick your hand under it.
Some of you might be mad at me—the water was hot and you have no idea why I asked you to scald yourself. (We’ll work through this.) Some of you, though, are going to be a little confused. You know the water was hot, but when you put your hand under it, it felt ice cold.
All together now, in your best Jerry Seinfeld voice: “What’s the deal with that?”
Feeling Spots
Our hands have a mess of sensory receptors that all receive different sensations. These receptors send signals to the brain to help us make sense of what we’re touching. We’ve got some receptors that receive sensations of cold (cold spots) and others that receive warmth (warm spots).
Neither of these temperature receptors pull double-duty. If you touch a cold spot with something hot, it’s still going to do what it’s supposed to do: send a cold signal. If you touch a warm spot with something cold, it’s still going to tell the brain that you’re touching something warm.
Mixed Signals
Neurologists call instances when these spots send the “wrong” signal in response to a stimulus paradoxical cold and paradoxical warmth. If you want to try another experiment (and you still trust me after the hot water thing), grab a pen and lightly poke the point around between your knuckles. In some spots it will feel cold, in some it will feel warm.
Of course, when you run your hand under hot water, the water touches both warm and cold spots. In cases like this, where the stimulus is strong enough, the receptors get confused and sometimes the wrong signal gets sent to the brain, even though both temperature receptors are being stimulated. Sometimes it only takes a second for things to correct themselves; sometimes it takes a few minutes.
How did the hot water experiment turn out for you?
If you’ve got a burning question that you’d like to see answered here, shoot me an email at flossymatt (at) gmail.com. Twitter users can also make nice with me and ask me questions there. Be sure to give me your name and location (and a link, if you want) so I can give you a little shout out.
Interesting! You have some points there!
posted by Rosabel on 8-27-2008 at 6:06 am
I remember an experiment in high school where you have a hose of warm water and a hose of cold water corkscrewing around each other. When you held them (together) it felt hot but apart they were warm or cold. That was fun.
posted by HunterJay on 8-27-2008 at 6:24 am
I remember when I was kid, probably around 10 yrs old or so, asking myself this very question. Now I know.
Thanks mental_floss!!!
posted by Justin L on 8-27-2008 at 8:47 am
OK, I have now officially poked holes practically through my skin and nothing. What and I doing wrong?
posted by GTT on 8-27-2008 at 9:38 am
People are staring at me, wondering why I’m poking myself at work…
This is pretty cool!
posted by Anthony on 8-27-2008 at 11:00 am
Ouch.
posted by mexichica on 8-27-2008 at 12:07 pm
Why doesn’t hot water feel hot when on your hand/arm/leg when you have poison ivy?
When you have poison ivy, putting it under HOT water, the water doesn’t feel hot at all, and it takes the “itchiness” away!
posted by Chris on 8-27-2008 at 8:58 pm
Your feet are much better at sensing temperature than your hands
posted by PartiallyDeflected on 8-27-2008 at 10:58 pm
I would conduct this experiment almost every night when I test the temperature of my shower water. I’ve always wondered about that, and now I know! Thanks!
posted by heather on 8-28-2008 at 1:22 am
What about when you have an itch in one spot of your body, but when scratching that itch, you feel it somewhere else?? Is that the same thing???
posted by colleen on 11-17-2008 at 12:45 pm
Hey colleen - I have wondered about that too - but even weirder, I think, is when I scratch my husband’s back and then I start getting itchy - check the website for an interesting article that may have something to do with it.
posted by Shmaem on 11-17-2008 at 1:10 pm
Yeah, that thing where you have an itch and when u itch it it pops up somewhere else, i get that too, it drives me crazy sometimes!
posted by mememememe on 11-17-2008 at 3:54 pm