i have tried the hot rock trick a couple times by just moving one from the fire to the tent vestibule with a heavy wire cooking grate.i let it warm up the tent,a Timberline,and get some of the damp out of it but even sitting off the dirt on a couple other warm rocks that i could move with leather gloves it went cold within 20 minutes and it was red hot when i moved it.i think this is a overrated camping trick.you would need a big fire and lots of good size rocks to get any effect.the old camping books show a guy sleeping away over nice warm rocks that are buried in the ground under him.ever try and dig a trench at a campsite?.unless you were at a beach it would take hours,and then more to collect the wood and rocks.i have put a candle lamp in a tent to do the same thing with good results and thats why i carry the weight of a small Gaz lamp,with one of those you have a hot tent in minutes.your water bottle filled with hot water in a sox works fine.the problem with the hot rock and other camping "tricks" is that people think they will be able to depend on them if need be and cut back on real gear,and in the end are cold and unhappy..if they survive