Well comparing the density of liquids is easy because liquid can change its shape so that we can see if we have the same amount. Well let us think about it. If we try and put a rock into a full cup of water, in order to go in it has to make some room for itself by pushing out the water that is in the space that it wants to take up. The rock is strong so it will take the space that it wants, the water will move out of the way. So let us take the big rock and put it in the brimful cup of water.
Lets pour the water that spilled out into a cup.
It has changed its shape but this amount of water is now takes up the same amount of space as the rock. So now we can compare the weights. Let us draw a seesaw. Which is heavier. Let us do this for all of the objects.