Energy is not easy to define.

Things have energy,

but you can't hold

a bushel of energy in your hands.

You can see what it does,

but you can't see it directly.

There are different types of energy,

but the differences between them

are manifested only in how they make stuff behave.

We do know that the total amount

of all the different types of energy in the universe

is always the same.

And, for chemists, two important types of energy

are chemical potential energy

and kinetic energy.

Potential energy is energy waiting to happen.

Think of a stretched rubber band.

If you cut it,

all that potential energy

gets converted to kinetic energy,

which is registered by you as pain.

Like a stretched rubber band,

chemical bonds also store energy,

and when those bonds are broken,

that potential energy gets converted

to other types of energy,

like heat or light,

or gets used to make different bonds.

Kinetic energy is the energy of motion,

and molecules are always moving.

They're not necessarily going somewhere,

though they could be,

but they are vibrating,

stretching,

bending,

and/or spinning.

Take methane,

which is four hydrogens

attached to a central carbon,

as an example.

Drawn on paper,

it's just a still tetrahedron.

But in real life, it's a jiggling mess.

The kinetic energy of molecules

is exactly the same type of energy

as the energy you have

when you're moving around,

except that you can be still

and molecules can't.

If you suck the kinetic energy

out of a group of molecules,

they'll move less,

but they'll never fully stop.

Now, in any group of molecules,

some will have more kinetic energy than others.

And if we calculate

the average kinetic energy of the group,

we'd have a number mathematically related to

temperature.

So, the more kinetic energy

a group of molecules has,

the higher its temperature.

And that means that on a hot day,

the molecules in the air around you

are spinning, stretching, bending,

and generally shooting around much faster

than on a cold day.

Now, hot and cold, by the way,

are relative terms.

They're always used to compare

one thing to something else.

So, on that hot summer day,

the air molecules have more kinetic energy

than the molecules in your skin.

So, when those air molecules crash into you,

they transfer some of their energy

to the molecules in your skin,

and you feel that as heat.

On a cold day,

the air molecules have less kinetic energy

than the molecules in your skin,

so when you crash into those air molecules,

you actually transfer

some of your kinetic energy to them,

and you feel that as cold.

You can trace the path of energy around you.

Try it at your next cookout.

You burn charcoal

and the release of that chemical potential energy

shows up as extreme heat and light.

The heat then makes the molecules

of your burgers, your hot dogs, or your vegetables

vibrate until their own bonds break

and new chemical structures are formed.

Too much heat and you have a charred mess;

just enough and you have dinner.

Once in your body,

the food molecules in your delicious,

or charred,

dinner get broken down,

and the energy released

is used to either keep you alive right now

or it's stored for later in different molecules.

As night falls,

the hot summer air cools

and the flow of energy into you slows.

Then, as the air reaches your skin temperature,

for the briefest of moments,

the flow stops.

And then it starts up again

in the opposite direction

as energy leaves the warmer surface of your skin

to return to the universe around you,

that energy, neither created nor destroyed,

but ever shape-shifting,

the chameleon phoenix of our physical world.