Part of the reason certainly is that we become inured to hot liquids in the mouth. However it is not the whole story. Firstly, we must bear in mind the difference in the effects of liquids at temperatures between say 40°C and 60°C. Practically any part of our skin or mucous membrane can stand 40°C indefinitely, whereas only a work hardened and can tolerate 60°C for more than a few seconds. Then there also are questions such as whether you have good thermal contact, how long the contact lasts, what part of the body is involved, and how much hot material there is. Your body's reactions to the various temperatures in that range are not simply quantitative. For example your hand can easily take a temperature of 45°C, but that is pretty hot for bathing in, and be careful of trying to wash your hair in anything so hot.

Now consider hot tea. 50°C might be uncomfortably hot for most people. It takes some time for water at 50°C to cause tissue damage, although contact soon becomes painful. (Please do not experiment too recklessly, it is surprisingly easy to scald yourself badly.)

Incidentally 50°C is just about the right temperature for any liquid you wish to apply to the skin for instant relief from jellyfish or stonefish stings, either of which can be life-threateningly painful. It also can still the pain of a bad cut. But still, beware of scalding.

When you drink hot beverages, you take in far smaller quantities than the liquid that you applied to your hands. It cools down far faster than say, the basin of hot water into which you dip your hand. The palms of your hand have far thicker skin than the backs, even if they are not calloused, in fact the skin of palms and soles of feet is so thick as to be a major challenge for taxidermists. It also reduces vulnerability to modest heat.

Then again, your saliva and mucus dilute small mouthfuls of hot tea, cooling it and helping to make it bearable as it moves nimbly over the tongue and down the gullet. In case you doubt the effect of this, you may wish to try drinking a lot of hot tea fast in large mouthfuls, but I strongly recommend against it. On the other hand, I doubt that you would try twice.

In the laboratory one cannot always afford the luxury of yelling and dropping a hot object thoughtlessly picked up. Those who have paid dearly for the necessary experience seldom do pick up hot objects carelessly, and do not easily burn themselves if they do. Workers at special skills such as in the processing of metals or glass, also need to develop such insensitivity (and sensitivity!) But the skills are not as cheap as they look. Amateurs beware!

Go cool,

Jon

When we learned to control fire and cook food, probabably about 1.5 million years ago, we gradually adapted to become smaller jawed and I'm wondering if our mouths became more 'heat-resistant' also. I would like to know if any comparative studies of differences between the cells that line our lips, tongues,throats,e.t.c. and those of other primates have ever been done?