To explain your kid such a complicated word as "density", you can use a visual experiment.
To make it we need some different liquids:
- honey
- gel for washing dishes
- water (with colour)
- plant oil
- alcochol
At first we read some info in the book: what density is and how to check it by an easy experiment.
My son didn't get at once what it is, and for 7 y.o it's really not easy, but when you take liquids and start to mix them, you see how it happens, and why they all have different places in one bottle.
So we took all these liquids and a big bottle.
Then we poured them all one after another using this very order (like I've mentioned above)
And as a result we see that they don't mix, they take different place in this multi-storeyed "house" of liquids, and the density is the reason of it.
It was a bright experiment that can explain a new word to a kid easy and fast🎇
