Good question. Let's first understand why
things are "hot". Everything is made of little
balls called atoms, few of which clump together to
form many small molecules. A thing feel hot
because of these little balls vibrating, bouncing
around - the faster they vibrate and bounce
around, the hotter the thing is; slower the atoms
the colder the thing is.
When fast moving atoms of the hotter thing
comes in contact and bumps into slower atoms or
molecules belonging to a colder thing, the hotter
thing cools down, while the colder thing heats
up. This is why a cup of hot coffee cools
down: the fast coffee molecules bump into slow air
molecules and become slower.
So, to keep things hot for a very long time,
one must prevent the "slowing down" of these atoms
and molecules. This can be done by isolating
the hot thing from everything around it. If there
are no slow molecules around to bump into, they
will remain hot for a long time. This is the
idea behind a "vacuum flask". Vacuum is
literally nothing. No atoms, no molecules in it.
In the flask, the hot liquid is separated/isolated
from air by a thick space containing vacuum (or
"nothing"). With nothing to bump into, the liquid
remains hot for a long time.
Another process through which things cool is by
emitting light (radiation). To avoid cooling
through this process, one must surround the liquid
by a mirror that reflects the emitted light back.
Be on the lookout for an "Infra-red camera" in a
science exhibition and trying imaging yourself
with it. You will see how much light your body