Let's play space tug o' war.  Tell you what, because it's your first time playing, you can have nearly everything in the solar system: planets, moons, asteroids, everything.  I will just have the sun.  Who do you think will win?  Let's see.  Hrrrrnnnnnnnnnt!  The sun wins.  Hands down.  "Solar system" means everything that moves around the sun.  It is not because everything in space thinks the sun is cool . . .

Everything you can see is made of stuff.  This is important to know if you ever feel like kicking something.  You can kick a balloon and it will sail a lot farther than a concrete brick.  There's a lot more stuff that makes up that brick than the balloon so it's a lot tougher to kick.  Just thinking about it makes my foot hurt.  Scientists don't use the word "stuff" when they talk about things.  They use the word matter to talk about all the stuff that makes up anything in the universe.  The balloon doesn't very much matter, but the brick has a lot in a very small space.  Mass is the amount of matter packed into something.  Just like the things in your classroom, everything you see in the sky has a different amount of mass.  Some of the stars you see at night have more mass than our sun; some have less.  Even though the sun isn't the biggest thing out there, it has A LOT more mass than Earth or the moon or all of the planets around us, all put together.  Yes, I knew that fact before we played tug o' war.  I wasn't cheating!  This is a science lesson.  You're learning.

If we could only line up our planets like this, we could see how much more mass our sun has.
NASAJayCeeH at lb.wikipedia, Public domain, via Wikimedia Commons

Everything that has mass will pull other things that have mass.  And they don't even need ropes to do it.  The more mass something has, the more other things are drawn to it.  Gravity is the force that pulls towards the center of anything with mass.  The Earth's gravity is what holds your feet to the ground and keeps you from sailing off into the sky.  The sun's gravity is what keeps the planets spinning around the sun and not flying off into space.  Space tug o' war may sound like a silly idea, but it's happening right now.  There just are no ropes.  

It was all going great until gravity took over.

When you play tug o' war, two teams pull in different directions.  This is what happens in space too, but it looks very different.  One side is spinning around the other super fast, and they both pull on each other.  To orbit means to move around something over and over.  If the sun wasn't pulling, the planets would fly off into space.  Since the Sun pulls them so hard, they stay moving around the Sun.  Imagine if instead of tug o' war players pulling a rope straight away from each other they were spinning in a circle.  No one would ever win, unless one team pulled a lot harder than the other.

The International Space Station has orbited the Earth your entire life.
NASA, Public domain, via Wikimedia Commons

So if the sun is always pulling on the Earth . . . and all the other planets . . . and everything else around . . . then why aren't they all falling into the sun to be burned up?  Why isn't the sun actually winning this game of tug o' war?  The sun never wins because all of the planets are always spinning around it super fast.  Even the strongest tug o' war players, like the sun with all of its mass, can't pull the other team toward them if they move fast enough.  Velocity is the speed and direction that something is moving.  The Earth travels with such high velocity that it's always rolling around the sun, like water going in circles around a drain that never gets pulled down.  It's moving too fast!  The Earth may not be a great puller.  But at least it's a good runner.  

Yes!  We're alive.  I realize you didn't know we were in danger.  And we weren't.  But still, it's good news that Earth isn't being pulled into the sun.  The sun has a lot of mass, or stuff, inside of it.  It has so much, in fact, that it bends space and makes things fall toward it with the force of gravity.  Lucky for us, the Earth is traveling at a fast enough velocity, or a speed in one direction, which keeps it from falling into the Sun in a big space tug o' war.  So I guess I did not win at space tug o' war.  It was a tie!  Just so long as your players keep spinning around my player, that is.


Universe Today.  "How Satellites Stay In Orbit"  Universe Today, 2011.  <http://www.universetoday.com/93077/how-satellites-stay-in-orbit/>