Your tongue sticks to cold metal because the cold metal steals heat away from your tongue and then your saliva then freezes solid inside the nooks and crannies of your tongue and so your tongues Saliva freezes to the metal.
Then you have to get that Saliva unfrozen to release your tongue.
You can really stick your tongue to a pole when it's cold enough outside.
Your tongue sticks to a cold metal pole because your tongue is warm and wet, and when you lick something like a frozen pole, icicle etc, the moisture from your tongue freezes and makes a connection to the frozen surface.
A metal pole exposed to freezing temperatures will quickly steal heat away from your tongue, faster than body heat can come to the tongue's rescue.
The result is that your saliva freezes solid inside all the nooks and crannies of your tongue.
You are stuck.
My brother did this when he was 7 years old and the pole he stuck his tongue too is still there on the corn of the street.
He stuck his tongue to a metal street sign pole on the corner about a few blocks from our house after a boy dared him too.
I tried to warn him but he wouldn't listen and stuck his tongue to the pole.
It was in the winter of 1995 when it happened and the temperature was below freezing.
I remember riding my bicycle back to our house to get help and my mom called 911 and the fire department came out and got it unstuck.
The firefighters poured a cup of cold water over the tongue to help loosen the tongue and eventually it came loose.
He never did that again.
If someone gets their tongue stuck to a pole which they shouldn't do you can loosen it by pouring a cup of water over the tongue until it looses and also have the child or person breathe on the pole as the warmth and moisture of the breath may help loosen the tongue from the pole.
If you can't get the tongue off yourself then call for medical help as they are equipped and trained to get things unstuck and other emergencies.