Blow up a balloon and slip the end of the balloon over the open end of a test tube, and place the closed end in a dewar full of LN2. When the balloon is completely deflated, lift the test tube out of the dewar and the audience can see your liquified breath in the test tube. The tube will frost up, but you can wipe the frost off with your fingers. Rest the test tube in a beaker, and as time passes, the balloon will inflate again. Please email me if you have any fancy ideas to be published about what else can be done with liquid nitrogen.
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Working with liquid Nitrogen may be dangerous.