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Say two people are walking down the street,

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and they bump into each other.

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They'll just shake it off and walk on.

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Sometimes that happens with molecules too.

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They just bounce off each other, and that's that.

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But what if two people were to bump into each other,

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and during that collision,

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one person's arm got severed

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and reattached to the other person's face?

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Now that sounds really weird,

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but it's similar to one of the many ways

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that molecules can react with each other.

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Two molecules can join and become one.

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One can split apart and become two.

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Molecules can switch parts.

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All these changes are chemical reactions,

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and we can see them happening around us.

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For example, when fireworks explode,

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or iron rusts,

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or milk goes bad,

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or people are born,

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grow old,

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die,

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and then decompose.

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But chemical reactions don't just happen willy nilly!

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Everything has to be right.

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First, the molecules have to hit each other

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in the right orientation.

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And second, they have to hit each other hard enough,

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in other words, with enough energy.

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Now you're probably thinking

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that a reaction just happens in one direction and that's it.

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Sometimes that's true.

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For example, things can't unburn

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or unexplode.

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But most reactions can happen in both directions,

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forward and reverse.

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There's no reason that our face-arm guy

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can't bump into armless girl,

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reattaching that arm back to its original socket.

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Now let's zoom out a bit.

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Now let's say that you've got

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a thousand people on the street,

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and all of them start with their limbs

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normally attached.

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At the beginning, every collision is a chance

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for Person A to transfer an arm to Person B's face.

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And so at the beginning,

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more and more people end up

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with arms attached to their faces or arms missing.

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But as the number of people with arm-faces

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and missing arms grows,

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collisions between those people become more likely.

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And when they bump into each other,

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guess what?

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Normal-appendage people are reproduced.

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Now the number of limb transfers per second forward

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will start high and then fall,

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and the number of limb transfers per second backward

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will start at zero and then rise.

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Eventually they'll meet,

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they'll be the same.

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And when that happens,

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the number of people in each state stops changing,

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even though people are still bumping into each other

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and exchanging limbs.

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Now how many people do you think

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there are in each state?

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Half and half, right?

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No, well, maybe.

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It depends.

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It could be 50/50,

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but it could be 60/40

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or 15/85,

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or anything.

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We chemists have to get our little, gloved hands dirty

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- ah, well, we're in a lab so not really dirty -

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to figure out what the actual distribution

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of molecules is.

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Even though each of limb transfers

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is a pretty dramatic event for the people involved,

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if we zoom out,

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we see population numbers that don't change.

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We call this nirvana equilibrium,

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and it doesn't just happen with chemical reactions.

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Things like gene pools

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and highway traffic show the same pattern.

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It looks pretty still from 30,000 feet,

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but there is lots of crazy stuff

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happening on the ground,

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you just need to zoom in to see it.