How are steam, smoke, and a hot air balloon the same?  Give up?  They all rise!  When something gets hot enough, it gets excited and goes up!  Lava shoots out of a volcano!  A popcorn kernel pops into the air!  A balloon fills with hot air and takes off into the sky!  Okay, here's another: what does a thermometer, bread fresh out of the oven, and a hot air balloon without a flame have in common?  They all sink down as they cool.  When something becomes cold enough, it falls back down.  What do these two riddles have to do with each other?  Well, it's this same power that moves the continents.

Hot steam goes up, cold water falls back down.

If only we had x-ray eyes!  The Earth's plates move way down below the crust, so we can't see how it happens.  We'll need to start by studying how things act up here on the Earth's surface.  Then we'll look at the few clues that come from the center to help us guess what goes on down there.  First off, we know that everything is made of very small parts that we can call particles.  The reason we can't see into the middle of the Earth is the same reason we can't see through a rock: all of its particles are super close together.  This is a good thing.  Otherwise, we might fall right through it.

Some things are a lot easier to move your hand through than others because their particles are spaced out.  Let's try to put our hand through different things.  It's easier to put your hand through air than water, easier to put it through water than pudding, and easier to put it through pudding than ice cream.  Plus, people might get mad if you put your hand in their ice cream.  Why is this?  Density is how close together the particles are in something.  As we know, the crust of our earth is very dense.  The magma under the crust, on the other hand, is not so dense, and is always moving.  Again, the layer in between is somewhere in between.

Under the Earth's surface is magma, which is very hot stuff that always wants to get out and up!  The layer between the crust and the liquid is not rock like the crust and not liquid like the middle of the Earth but something in between.  When something is a fluid, its particles are spaced far enough apart for them to move.  This means you can move your hand through that fluid!  Air, water and magma are all fluids  . . .  just please don't try to move your hand through magma.  The inside of the earth may look and act like rock in order to hold us up, but it's also moving like a fluid.  When something becomes hot, all the particles inside of it become more excited.  They start to bounce around and spread farther away from each other.   With all this new space, the thing itself becomes lighter and takes up more space.  It needs somewhere to go!  Often, that place is up.

Now let's watch some bubbles.  The next time someone is boiling water, watch as the water becomes excited.  Little bubbles form at the bottom of the pot and then rise to the surface.  Soon the water itself will start to get hot and drift out as steam.  This also happens in dense things, but since they are so packed together, the molecules move less.  Think of our bread dough again.  When you put it in the oven, it becomes hot and it rises.  It grows bigger as it pushes out, taking up more space and giving the particles more room to move.  Convection is when hotter things move up through a fluid like air or gas.  So if the stuff in the center of the Earth gets hot, and hot things want to rise  . . .  where does it go?

Let's watch and see what bubbles up here.

Nowhere!  The Earth's crust is blocking it.  So, it will rise, reach the colder crust of Earth and then cool to fall back down, and then it will rise again.  The fluid moves in circles, and that moves our crust.  A current is a fluid that slowly moves in one direction.  An example of this is a river of water going down a mountain, except currents can go up when they get really hot.  We believe there are a few of these rising and falling circles beneath our crust.  As the tops of these circles rise and then fall again, they push and pull the continents around, slowly but surely.  It's kind of like how a treadmill will push you off if you don't keep running.

Watch out, some rivers have very strong currents.

Convection currents are the things that move our land around.  Without these driving circles, our Earth would never shake, but it might also mean that there would be no life to feel it shake!  And now you know one of the big reasons our Earth is different than any other place that we know of in space.

References:

Science For Kids.  "Hot Air Rises."  Kidipede, 2011.  <http://scienceforkids.kidipede.com/physics/weather/hotairrises.htm>

Plate Tectonics.  "How Plates Move."  Plate Tectonics, 2013.  <http://www.platetectonics.com/book/page_4.asp>

unomaha. "Relationship between mantle convection currents and plate motions?" Unomaha, 2012. <http://maps.unomaha.edu/Maher/plate/week13/mechanisms.html