# Frozen children

A few weeks ago, my friend Amanda, an elementary school teacher who runs a children’s camp during the summer break, suggested that it could be fun for me to come into the camp one day and do some science demonstrations for the kids. I jumped at the opportunity, despite (or perhaps because of) the fact that I am a purely theoretical physicist and my day-to-day work only involves whiteboards and computers at Caltech. Most of the children attending the camp are relatively young (7-9 year-old kids) so, rather than setting out to give a science lesson, I viewed it as a chance to do some fun demonstrations and get these kids excited about science! Besides, I had an ulterior motive; it was a great excuse to acquire, and play with, liquid nitrogen (LN$_2$) from a Caltech lab (of which most of the IQIM labs have copious supplies). LN$_2$ is great for demonstrations; this stuff is awesome! At a temperature of $-321^{\circ}\,F$ (for reference, the coldest temperature ever recorded on the surface of the Earth is $-128.6^{\circ}\,F$), it behaves in ways unlike anything that most people have ever seen. I convinced my friend Carmen, a postdoc in astronomy at Caltech, to come along and help out. Here, I thought I would share my experience, as well as some of the things I learned about handling LN$_2$.

Carmen watches on as I pour the liquid nitrogen into the beaker, which boils like crazy until the beaker is chilled. The white gas is actually water vapor condensing from the air; nitrogen gas is transparent (think of your ability to see through air, which is mostly nitrogen gas).

Liquid nitrogen volcano! All it takes is a little water added to the liquid nitrogen dewar.

Crime and punishment: As anyone who has seen Terminator 2 knows, objects that are pliable at room temperature become brittle and can shatter when reduced to cryogenic temperatures (including robotic assassins from the future). Thus I devoted a significant amount of the demonstration time to freezing and breaking everyday objects, including flowers and rubber toys. The flowers were particularly spectacular, shattering like glass into a multitude of pieces when struck against the table, providing a good deal of entertainment for the audience as well as myself. Hasta la vista, baby. I also froze several pennies, which then became brittle enough such that Carmen was able to shatter them with a few taps from a hammer. Incidentally, destroying US currency is illegal (which is why I had Carmen do it instead of doing it myself). I informed the children of this fact and asked who among them thought that Carmen should go to prison for her crime. A quick vote revealed that the majority of the children thought that she should be behind bars. Sorry Carmen, maybe the next field trip for the camp can be to visit you in prison?

A flower, freshly pulled from the vat of liquid nitrogen, prepares to make the ultimate sacrifice in the name of science.

After having frozen a variety of objects, one of the children asked me whether you could freeze people with it. I told the kids that this is something that I always wanted to try, but that I had previously lacked a volunteer, to which an enthusiastic boy jumped up and responded, “freeze me, freeze me!” I asked whether he wanted to be frozen 5 years, 10 years, or longer? He said he would like to be frozen until the end of the world. One must admire his dedication! Before attempting to freeze him, I told him that it would be prudent for me to try it on something less likely to have litigious relatives. To this end a strawberry, a peach and a plum were submerged in LN$_2$, and then removed and allowed to slowly thaw. They ended up melting into gelatinous blobs; clearly some kinks in my cryogenic freezing and revival process need to be resolved before I graduate the approach to small children.