Why does warm water feel scalding to cold hands?

Chris - This is called adaptation and under normal circumstances, the nerve cells which are carrying sensory information from our peripheries into the core of the body, into the central nervous system, to inform the brain as to what the environment is like, they are very rapidly adapting.

What this means is that they are largely interested in how things are changing. They're not so interested in the static situation. That's because if you didn't have something that quickly got bored, and stopped signalling the absolute level of something, then you'd suffer from sensory overload. So they tend to signal change.

Now if you have got your hands very cold, then there's very little increase in the temperature in your hands and, therefore, the nerves that signal warmth are largely very inactive. And the ones that signal cold are firing off saying, "It's cold and I'm getting colder."

When you then put your hands indoors or under warmish water, the amount of firing in the warm fibres now goes through the roof because where they were previously not very responsive, they've gone from going, "I'm very, very cold so I'm not going to react" to suddenly, "Wow! The temperature is increasing very, very rapidly."

And it's actually the change that they're interested in, and so they fire intensively, and that's why you experience this sensation that things are much, much hotter than they are.

Dave - There's also a lovely experiment you can do with this, completely confusing your body. If you put one hand in a very hot water, one in a very, very cold then put them both in the middle, it's entirely confusing. One hand feels hot, one feels cold in the same water.

Chris - It's actually in our book, Crisp Packet Fireworks. We actually explained how to do that experiment if you want to have a go.