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Step 1: Warm Beer.
Step 2: The Setup
Step 3: How Its Done
In the video it is obvious that the thermometer is measuring the air temp around the beer, but if you watch closely you can see the frozen gases spraying onto the can and frosting it. If you think this is some kind of hoax then hold a can of duster upside down and spray into a glass of water... it will start to freeze.
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