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DANGEROUS DEMO! Can't get liquid nitrogen? Then make your own -80C version by using Dry Ice. Note that this demonstration involves several major safety hazards, and should only be performed by skilled educators.
Note that dry ice comes from companies listed in your local yellow pages. Some welder supply stores carry it. In Seattle, you can find it in the seafood section of QFC neighborhood grocery. Ask for dry ice pellets rather than slabs, so you won't have to chop it up yourself.