I did not believe them but they were right!
These have amazing heat retention!  After a week of testing I can say that after even 20Hours there is enough residual heat to know the difference between one of these that was under my blanket and one that was of rubber, filled with the same water, heated and filled at the same time. The rubber one I have from Walmart lasts perhaps 4 to 5 hours with heat....and these are lasting so much longer with much less water.<br /><br />I wanted these for camping, which will be tested outdoors for a week in late February, at a Historical Reenacting Event in my period tent. I am certain these will be the best i can get. I tested using half tap water and half from my kettle filled 3/4 as stated in directions. I am finding that even as little as 2 cups of water will work (which will be important for camping). I have health issues with cold intolerance and this and my heater should do  the trick so I am not up all night shivering because I cannot warm up my equipment enough to keep my body temp stable. Its a health condition.<br /><br />Last night I tried under 2 cups water and it was still at least body temp or slightly warmer 12 hours later. I did make myself a insulated cover from a cheap cotton shop towel I bought at costco in a multi-pack. It helps insulate too as well as protecting from getting skin too hot (and later my inflatable mattress when camping). The opening is a better angle and bigger opening than the others I have seen or used before. It makes it much easier to fill and take the excess air out. I also am allergic to latex so bonus that these are silicon.
Rating: 5 stars