* Density is mass per unit volume. Think of it as how much stuff is packed into a given space.
* Closer particles mean more mass in the same volume. When particles are tightly packed, there's more matter squeezed into the same space.
* More mass in the same volume leads to higher density.
Example: Imagine a box filled with marbles. If you squeeze the marbles together so they are closer, you'll have more marbles in the same box. This means the density of marbles in the box has increased.
