I bet most of you think it's:

"What goes up,

must come down!" Is that right?

Well, sorta, but not really.

Technically, the law of gravity is an equation.

It is: F = G x M1 x M2 / R^2,

where G is the universal gravitational constant,

M1 and M2 are the masses of the two objects,

and R is the distance between them squared.

That was easy, right?

Probably not. What does this actually mean?

Well it means that - well, everything is attracted to everything else.

What I mean by that is if you have two objects, any two objects,

they are attracted to each other. OK.

Let's try and wrap our minds around this.

What happens when you drop a rock off a cliff?

It falls to the earth. Right?

Well, yes, but something else happens.

You see, the law of gravity says that both objects,

the rock and the earth, are attracted to each other.

This means that the rock falls towards the earth,

and the earth falls towards the rock.

Wait a second - you mean to tell me that if - the earth falls up to meet the rock?

Yes, that's exactly what I'm saying.

And the reason you don't see the earth fall up to meet the rock

is because the objects move towards each other proportionate to their respective masses.

The earth is much much much more massive than the rock,

so it moves a very very small distance,

and the rock is much less massive, so it moves farther

with respect to the earth.

Maybe a better way to understand gravity is to take two teenagers

in spacesuits, and place them far out in space - away from all the planets and the stars.

It turns out they will be attracted to each other.

I'm not talking about that kind of attraction.

See, they have mass, and since they have mass,

they will move towards each other.

They are attracted to each other.

Maybe one more thing might help. Have you ever played with two magnets?

You know, the magnets with the north and the south poles?

When you take the magnets and put them closer to each other,

they move together. They are attracted to each other.

And the closer they are, the stronger the attraction.

Think of the mass of the object like the strength of a magnet

and the distance between the objects like the distance between the two magnets.

Now understand, I'm not saying that gravity and magnetism are the same,

they just behave in a similar way.

Let's think of one other thing - astronauts.

You know, astronauts, they weigh less on the moon

than on the earth. Why is that?

Well you see, the moon is less massive than the earth.

Therefore it has a smaller gravitational pull on the astronaut.

It's like the moon is a weaker magnet.

They aren't as attracted to each other.

Distance also plays a role. Think back to playing with a magnet.

The pull of the magnets towards each other

are stronger when they are closer together. The same is true of gravity.

For example, the sun is the most massive object near the earth.

It dictates most of the gravitational forces in our solar system.

It is very very massive. But it is relatively far away,

so even though the sun is a much stronger magnet, so to speak,

it is a long ways away.

Therefore the attraction isn't as strong.

So let's look back at that law of gravity.

The equation: F = G x M1 x M2 / R ^2.

You see the force of gravity is equal to a number.

That's that universal gravitational constant G

times the mass of object one, times the mass of object two.

Think of M1 being the mass of the sun

and M2 being the mass of the earth.

And then we divide by the distance between them squared.

This determines the force of attraction between the sun and earth.

You could just as easily plug in your mass

and the earth's mass and the distance between you and the center of the earth,

and find out how much you are attracted to the earth,

and the earth attracted to you.

So, what's gravity?

Everything is attracted to everything else.

Everything. Oh, one last thing,

just to make you wonder. What causes gravity?

Why are two objects with mass attracted to each other?

Well, the answer is -

We don't know.

The cause of gravity remains a mystery to scientists.

We don't really know conclusively what causes gravity.

It is one of the great mysteries of science.