Salt doesn't actually make water *heavier* in the sense of increasing its mass. Instead, it increases the *density* of water. Here's why:

* Density is mass per unit volume. So, a denser substance has more mass packed into the same space.

* Salt dissolves in water. When salt dissolves, its ions (sodium and chloride) become surrounded by water molecules. This disrupts the normal structure of water molecules, making them pack closer together.

* Closer packing means higher density. Since the same amount of water now occupies a smaller space, its density increases.

Think of it like this: Imagine a box filled with marbles. Now imagine adding some sand to the box. The sand fills in the gaps between the marbles, making the contents of the box more compact. The box hasn't gotten heavier, but it's denser because more stuff is packed into the same space.

So, while salty water has a higher density, it doesn't necessarily have more mass than fresh water. It just has the same mass packed into a smaller volume.