How To Create A Mini Refrig.

Doing some research as of late on basic survival stuff, I came across this interesting site that details how to make a small refrigerator-like container. Called the Pot-in-Pot refrigerator, you simply need a couple of clay pots (one large, one small), sand, water, some cloth, a thermometer and something to plug the hole of the pot (if there is one). I'm curious to know what temperature was achieved.