Captain, there's a meteor headed right at our ship.  That's not the worst news.  Our instruments are broken so we didn't see it until now, we cannot tell how big it is, and we cannot move away.  All we can do is sit and wait for it to come.  But I looked out of the spaceship's window, and the meteor is either very close and the size of a pea . . . or it's very far away and it's the size of our ship.  Yes, yes, I have become too dependent on our instruments.  You can fire me if we live through this. 

One thing is for sure.  This meteor is going to hit us.  A collision is when one object runs into another.  One of them might be moving while the other is sitting still.  They can both be moving and run into each other.  Whatever was happening before they hit, there will be a change in what they're both doing once they touch.  It can be as small as a balloon bouncing off of your head or as big as the Titanic crashing into an iceberg.  Let's hope this change is more like that first one. 

Did you say a meteor hit you?

No matter how big the meteor, it is going to change our speed . . . either a lot or just a little.  When things move, they have energy that they can pass on to things they hit.  A transfer of energy is when the movement of one thing is passed to another thing when they touch.  If the meteor is big and flying quickly and hits our ship, it could send us flying the other way.  If it's the size of a pea, it will send us moving away very slowly.  In which case this talk will not have had much of a point.  Yes, I hope that's true too.

A pea sized meteor. Now that's something you don't see everyday.

No matter how big it is, this space rock is coming at us.  This happens once it hits us, then something else will happen.  In science, we use a word to talk about the first thing that happens.  An action is the cause or the first thing that happens right when two objects hit.  In this case, the action is the meteor hitting the ship.  This makes other things happen and change.  Once it hits us, something else will happen because it hit us.  If it's the size of a pea, the meteor will give a little bit of energy to the ship.

If the meteor is big . . . that's when we will need to worry.  Our ship will have a lot of energy given to it and it might send us moving fast . . . and in many different pieces and directions.  A reaction is something that happens in response to something else.  In this case, the reaction will be what happens to the spaceship once it is hit.  This could be as small as getting a tap from a speeding space pea.  Or it could be as big as bouncing off of a big meteor in the other direction.  Look on the bright side.  If it is big, and we live through it, we could get a big boost back home.

This is a crater from a meteor that hit Earth in the Arizona desert. Imagine if it hit your spaceship.

A meteor is coming at our ship.  It could be big.  It could be small.  One thing is sure, we are going to have a collision.  It's going to crash into us.  When it does, it will transfer its energy, or the speed it was flying, into our ship.  It has an action, or a thing it's doing, and that will pass to us as a reaction.  It could slow us down or send us flying in the other direction.  Oh, look!  Our instruments are working again.  Well . . . that's embarrassing.  It turns out the meteor I saw was just a doughnut crumb on the spaceship's window . . .  Please don't fire me.