Liquid nitrogen is one of the coldest substances on the earth. It has a temperature of -321 F (-196 c) and can freeze and shatter anything with which it comes into contact.
The extremely low temperature of the liquid can cause severe frostbite, but you can stick your hand in liquid nitrogen at -321 F degrees and still be unhurt.
How can a human body sustain this low temperature?
The answer lies in something called the Leidenfrost effect, in which a liquid, while coming into contact with something hotter than itself, forms an insulating layer of vapor between the liquid and the hot surface to prevent the liquid from boiling immediately.
The vapor layer prevents the heat transfer between the two and provides instant protection to the object, such as a hand against the hazardously cold liquid nitrogen. And the hand can be dipped in the liquid nitrogen again and again, though each time for only a split second.
You can see this crazy footage of the Leidenfrost Effect below: