The short answer is that the water on your tongue freezes solid between the skin on your tongue and the cold metal. For this to happen the temperature of the metal must be below 32 oF, otherwise the water cannot freeze. The lower the temperature of the metal the quicker your tongue will stick. Even your hand may stick to cold metal if your hand is a little sweaty – like when you pull your hand out of a warm glove.
Now, for the finer points. If you put your tongue or hand on a cold piece of plastic or rubber, even if it is very cold, it is unlikely to stick. I’ve done that experiment many times and even when the plastic or rubber is minus 300 degrees (much colder than it gets even at the earth’s poles in the winter – about the boiling point of liquid nitrogen) my hand or tongue does not stick. Why is that?
The answer involves a property of materials called thermal conductivity. Materials that have high thermal conductivity will transfer lots of heat from a higher temperature to a lower temperature. To be exact, the heat conducted away from the higher temperature material (in this case your tongue) to the lower temperature material (the metal) is equal to the product of the thermal conductivity times the temperature difference between the two materials. For the water to freeze on your tongue, heat must be extracted from the water to lower its temperature to the freezing point (32 oF). But your tongue is warm and your body continuously supplies heat to it through your circulating blood. So the material you touch must have a high enough thermal conductivity to extract heat faster from your tongue than it can be supplied by your blood.
So you guessed it! Metals have a high thermal conductivity, high enough to make the water freeze on your tongue. But plastics and rubbers have much lower thermal conductivity (about a hundred times lower) and cannot take enough heat away from the water on your tongue to make it freeze, even if these materials are very cold (a large temperature difference between it and you).
So if you are going to put your tongue on something cold, first be sure that it has a low thermal conductivity. By the way, if a popsicle or the ice on a pond is cold enough, your tongue will stick. Ice has a thermal conductivity between that of metals and of plastics.
Frank J. DiSalvo
Ph.D. Stanford University
Wife, Barbara, 2 Daughters, 4 grandchildren
Fishing, swimming, traveling