Ineed with density i don't get it at all

Density is the ratio of mass and volume.
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To fill the problem imagine that you have 1 liter of water and 1 liter of feathers.
For sure 1 liter of water has greater mass. It because water has greater density. In another words, there is more molecule in the same volume.

Which is heavier, water or rock ?

Did you say rock ?  Oh yeah ?  Are you sure ?  When I asked the question,
I was thinking of the little rock in my pocket, and a whole swimming pool
full of water.  So the water is heavier.

That's not fair, is it !   When you were figuring out which one is heavier, you
thought about the SAME AMOUNT of each one ... like one bagful, or one jarful.

And that's what 'density' means.  It's the amount of mass of a substance,
but always the same amount of it.  The same amount is always 1 cubic
centimeter, and the density is always how many grams of mass there are
in that much of it. 

That way, you can compare any two substances in the world.  When you
talk about their densities, you always know that you're talking about the
same amount.