heat can create many things like solid liquid and gas see the heat in a solid are able to more around in their own space but in a liquid their all jumbled up they no space to move around but in a gas they have a lot of space theirs only like three or four of them .
I know what your thinking mass and weight are the same thing well think again mass is way different you know how gravity keeps you on earth well mass keeps you down too if you go to the moon you still weight the same but your mass is different
the pic. bellow is an example of a conductor of electricity. for example you know how you use a institution cored to plug stuff in well that's a conductor of electricity when you plug it in the energy flows through for the item to turn on.
An example of Density is the pic. below you you take a cube and you put it in water and see if it sinks or floats the water push or pull the object down or up it all depends on how heavy or light the object is.