First, you need to know what heat is. Heat is the rattling,
wiggling motion of all the atoms that make up a substance. It's a
form of motion energy, but special because the solid as a whole isn't
going anywhere. How is that possible? Imagine a large crowd of
people standing around, in line, perhaps, for concert tickets. They
could be standing quietly or shoving and milling about, even though
the crowd isn't going anywhere. It's like that with solid objects,
which are really vast "crowds" of individual atoms, that may be quiet
(cold) or may be milling around excitedly (hot).
A hot (jumpy) solid contains more energy per unit volume than a cold
(quiet) solid. If we put a hot solid next to a cold, then some of the
energy in the hot solid will flow into the cold solid. This is called
the flow of heat.
OK, now we need to think about how heat flows from something hot
(your hot chocolate inside your thermos) to something less hot (the
atmosphere). There are...