ok so I am trying to make a box that will absorb heat and hold it for a long time. (cheaply) But It has been a while since my last thermodynamics discussion and I keep confusing myself. specific heat I know is the value that of heat it takes to raise temperature. But does it also tell how well that material can hold heat? or is that thermal conductivity or something else all together. or maybe I don't need to know the theory, maybe I can just ask. If I have a box I make out of steel, what is something cheep I can put n the box to help it hold heat. I am thinking sand but it has a low specific heat so I don't know if that is the right answer. Water will not work.