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Energy is not easy to define.

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Things have energy,

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but you can't hold

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a bushel of energy in your hands.

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You can see what it does,

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but you can't see it directly.

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There are different types of energy,

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but the differences between them

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are manifested only in how they make stuff behave.

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We do know that the total amount

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of all the different types of energy in the universe

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is always the same.

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And, for chemists, two important types of energy

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are chemical potential energy

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and kinetic energy.

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Potential energy is energy waiting to happen.

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Think of a stretched rubber band.

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If you cut it,

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all that potential energy

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gets converted to kinetic energy,

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which is registered by you as pain.

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Like a stretched rubber band,

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chemical bonds also store energy,

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and when those bonds are broken,

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that potential energy gets converted

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to other types of energy,

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like heat or light,

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or gets used to make different bonds.

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Kinetic energy is the energy of motion,

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and molecules are always moving.

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They're not necessarily going somewhere,

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though they could be,

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but they are vibrating,

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stretching,

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bending,

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and/or spinning.

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Take methane,

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which is four hydrogens

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attached to a central carbon,

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as an example.

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Drawn on paper,

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it's just a still tetrahedron.

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

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The kinetic energy of molecules

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is exactly the same type of energy

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as the energy you have

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when you're moving around,

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except that you can be still

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and molecules can't.

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If you suck the kinetic energy

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out of a group of molecules,

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they'll move less,

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but they'll never fully stop.

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Now, in any group of molecules,

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some will have more kinetic energy than others.

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And if we calculate

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the average kinetic energy of the group,

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we'd have a number mathematically related to

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temperature.

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So, the more kinetic energy

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a group of molecules has,

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the higher its temperature.

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And that means that on a hot day,

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the molecules in the air around you

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are spinning, stretching, bending,

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and generally shooting around much faster

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than on a cold day.

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Now, hot and cold, by the way,

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are relative terms.

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They're always used to compare

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one thing to something else.

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So, on that hot summer day,

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the air molecules have more kinetic energy

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than the molecules in your skin.

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So, when those air molecules crash into you,

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they transfer some of their energy

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to the molecules in your skin,

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and you feel that as heat.

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On a cold day,

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the air molecules have less kinetic energy

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than the molecules in your skin,

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so when you crash into those air molecules,

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you actually transfer

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some of your kinetic energy to them,

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and you feel that as cold.

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You can trace the path of energy around you.

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Try it at your next cookout.

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You burn charcoal

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and the release of that chemical potential energy

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shows up as extreme heat and light.

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The heat then makes the molecules

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of your burgers, your hot dogs, or your vegetables

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vibrate until their own bonds break

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and new chemical structures are formed.

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Too much heat and you have a charred mess;

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just enough and you have dinner.

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Once in your body,

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the food molecules in your delicious,

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or charred,

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dinner get broken down,

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and the energy released

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is used to either keep you alive right now

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or it's stored for later in different molecules.

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As night falls,

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the hot summer air cools

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and the flow of energy into you slows.

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Then, as the air reaches your skin temperature,

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for the briefest of moments,

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the flow stops.

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And then it starts up again

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in the opposite direction

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as energy leaves the warmer surface of your skin

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to return to the universe around you,

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that energy, neither created nor destroyed,

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but ever shape-shifting,

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the chameleon phoenix of our physical world.