I don't see this mentioned elsewhere, so I'll mention it here. Liquid N2 is benign and hard to harm yourself with, apart from frostbite. But it is colder than

I don't see this mentioned elsewhere, so I'll mention it here. Liquid N2 is benign and hard to harm yourself with, apart from frostbite. But it is colder than the boiling point of oxygen, and if you leave open containers of it around, or chill empty glassware, you can condense O2 out of the air. One dangerous scenario is where you chill an empty flask that has organic residue in it, and liquid oxygen condenses onto that. It can go boom. It's not easy to hurt yourself this way, but something to think about (while running at top speed in the opposite direction) if you see pale blue liquid in an N2-cooled apparatus. On the bright side, you can make fantastic ice cream by pouring liquid N2 into a bowl of heavy cream with walnuts and sugar added. The crystals are super tiny so the ice cream has a fantastic texture.