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What I want to do in this video

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is talk a little bit about maybe why the plates

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are actually moving in the first place

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And nothing I'm talking about in this video

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has been definitively proved,

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this is just kind of the current

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thinking, the leading thinking

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on why plates are actually moving.

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Although we haven't seen

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the definitive evidence yet

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and there's probably a combination

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of a bunch of things.

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Now before we even talk about plates,

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let's just talk about convection.

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And you might already be familiar

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with the term,

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but just in case you're not,

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let's do a little bit

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of review of convection.

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So let's say I have a pot

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over here, so that is my pot

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and it contains some water

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So I have water in my pot

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and let's say I only heat

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one end of the pot

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So I put a flame

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right over at that end of the pot.

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So what's going to happen?

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Well the water,

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the water that's right over

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the flame is going to be

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warmed up more then

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any of the other water

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so this water

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is going to get warmed

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but when it gets warmed,

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it also becomes less dense

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when you have a fluid

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if you warm it up

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the molecules ar vibrating more

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they have more kinetic energy

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they're going to bounce

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further distances away from each other

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they will become less dense

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and if you have something

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that's less dense

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and it's surrounded by

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things that are more dense

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and we're dealing with a kind