BT BB 19a(45) :משום דמחממי חאים ומקררי קאיר‏ : because (sand surrounding) heated objects heats up and (sand surrounding) cooled objects cools down [i.e. it acts as an insulator and is relevant only to the Sabbath]
See the entire entry for ḥmm vb. to be hot

BT BB 19a(45) :משום דמחממי חאים ומקררי קאיר‏ : (sand around) heated objects get warm and that of cool objects get cool †
See the entire entry for mḥmm n.m. hot object