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You've seen what happens when you put flowers in liquid nitrogen. They freeze to the point they shatter when struck. So what on earth would cause someone to stick their hand in liquid nitrogen? Science, of course.
This video from YouTube's NurdRage demonstrates just what happens when you stick your hand into a bucket of liquid nitrogen (something you should NEVER try, by the way). What's amazing is his hand doesn't freeze upon contact, as one would expect.
The reason behind it is the Leidenfrost effect, a phenomenon in which a vapor forms between a liquid and a hot surface, causing the liquid to bounce off the heat (similar to water droplets rolling around on a hot skillet).
Since the temperature of liquid nitrogen ranges from -210oC to -195.8oC (-346oF to -320.44oF), a human hand seems boiling hot in comparison - which explains why it wouldn't freeze immediately. Here's what NurdRage explained in detail:
"The Leidenfrost effect is the formation of a gas barrier between a hot surface and a boiling liquid if the temperature difference is great enough. This gas barrier greatly slows the heat transfer between the two and allows the liquid to last longer and consequently the hot surface to remain hot longer. This effect can be seen in a frying pan as it's being heated. At first the water quickly boils as it's dropped in but at a hot enough temperature the Leidenfrost effect takes over and makes the water skate around the surface lasting a very long time."
Watch the experiment below but please DO NOT TRY THIS AT HOME!