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 three ways:
(1) CONDUCTION. If the hot solid is touching the cold solid, then
jumpy (hot) atoms can bump against their quiet (cold) neighbor atoms,
and if they do the hot guys get quieter and the cold guys heat up.
Then the newly-hot-used-to-be-cold guys start heating up *their*
neighbors, in turn, and so forth until the heat flows deep into the
cold solid.
(2) CONVECTION. If the hot solid is not touching the cold solid,
there will usually be some third material between the two. If the
third material is also a solid, we just have conduction from hot to
third material, and from third material to cold solid, category (1),
already dealt with, phooey.
But if the third material is a liquid or gas, something new can
happen. Suppose for the sake of argument the hot solid is on the
bottom, the cold on top, and a liquid in between. This describes
things like a saucepan on the stove or the inside of the Earth. A hot
atom will jostle a neighbor cold atom in the liquid, heating it up.
When a layer of liquid atoms near the hot solid are all jumping
around, they take up more room, as you'd not be surprised to hear.
That is, the liquid expands. When it expands, it weighs less per
fluid ounce, quart or gallon than the cold liquid above it. Now, you
know how light fluids (like oil) float to the top of heavier fluids
(like water)? And hot gas (in a hot-air balloon or from a smokestack)
rises up through colder gas? Same thing happens here. The hot fluid
rises up to the surface, where it meets the cold solid, and gets
cooled off. Then it sinks down, and starts the cycle all over again.
With convection you get very efficient cooling because the hot
solid always has fresh cold material coming in. Almost all modern
cooling systems -- for your car, refrigerator, A/C, etc. -- use
convection of a ``refrigerant.''
You also get strong currents in the fluid. That's why there are
currents in the ocean (which is heated by the Sun and the sea floor),
currents (winds) in the Earth's atmosphere, which is heated by the
Earth's surface, and currents in the Earth's mantle (it's heated by
the Earth's core further inside) that lead to volcanos (when an
up-traveling current of hot rock bursts up through the crust).
(3) RADIATION. A solid will also shake loose on occasion a little
low frequency light, and when it does some energy escapes as the
(invisible) infrared light, and the atoms cool down a bit. Radiative
cooling is not very efficient, but when there is no material at all
between the hot and cold solids, it's all there is. The Sun heats the
Earth by radiation across the empty space between.
Now, how can we STOP heat flow, so your hot chocolate stays hot in
the thermos? We do it three ways. First, we put an empty space
between the hot and cold solids. So we make a bottle and put it
inside a slightly bigger bottle, seal the gap, and suck out all the
air. This is the thermos bottle. With no material in between,
conduction and convection do not work. The bottle's made of glass
because it's easy to seal glass up so there are no leaks (you just
melt it a little). Finally, we try to slow down radiation by painting
the inside of the outer bottle silver, so it reflects any radiation
that falls on it from the inner bottle. Hence, a thermos bottle has a
mirror (shiny) surface.