The temperature of regular stuff is basically just a measurement of jiggliness of the atoms
and molecules that make that stuff up.
More jiggling, higher temperature.
Less jiggling, lower temperature.
Of course, when something's at a high temperature, it feels hot, and when something's at a low
temperature, it feels cold.
If you touch a piece of metal and a book that have been sitting in your fridge, the metal
will feel much colder than the book - Derek of Veritasium did a great video on this, but
you really have to try it for yourself to believe it!
The metal and the book are honestly at the same temperature as measured by a thermometer,
but the metal FEELS colder.
This isn't just a trick of the mind, though - we experience the metal as "colder" than
the book for a very physical reason: metal is a conductor, and paper is an insulator,
so the ENERGY, or jiggliness of the molecules in our hands, is absorbed more quickly by
the metal than by the book.
Even though the book and the metal are at the same temperature, the metal causes the
temperature of our hands to go down faster, and thus, we experience the metal as being
colder - because the temperature of our hands is what we really feel.
It's like how, technically, a mercury thermometer really only measures its own temperature and
you can only indirectly measure temperatures of other things by putting them in thermal
contact with it.
Similarly, the thermoreceptive nerves in our skin can only directly measure the temperature
of the skin itself and not of anything else.
So when we touch something, we don't feel its temperature, but rather, we feel its effect
on our skin: that is, how much and how quickly it transfers thermal energy - that's the jiggling
of molecules - to or from us.
The capacity to transfer thermal energy is also why a blast of steam from your stovetop
can feel so much hotter than a blast of hot dry air from your oven, even though the oven
has a higher temperature: water vapor transfers more molecular jiggling to your skin than
air by itself.
In fact, it's tempting to say that "hot" and "cold" are fundamentally different concepts
from "high temperature" and "low temperature", even though we usually use the words interchangeably.
"Hot" really means "it gives off a lot of energy" while high temperature means "it has
a lot of energy" - and as anyone who's tried fundraising knows, just because somebody has
a lot of something, doesn't necessarily mean they give a lot
of it away.