WHEN YOU HEAT SOMETHING THE MOLECULES GET HOT, EXPAND SO IT GETS LESS DENSE
WRONG!!
Well.....when you heat something it DOES expand and become less dense, but it is not because the molecules expand. Many people confuse the behaviour of the molecules themselves
with the overall substance. A molecule is a molecule is a molecule. A molecule cannot get hotter and it does not expand. When we heat a substance the extra (thermal) energy is stored in the substance as kinetic (and potential) energy of the molecules...that is: the hotter something is,
the faster its molecules are moving. So the only thing that molecules can do is move faster or slower.
When the molecules are moving faster (when the substance is hotter) they collide with or interact with their neighbours more violently...this means that all the molecules push each other a bit further
apart so the substance becomes less dense.