While they seem like very different things, air, helium gas, salt water, and copper wire all have one thing in common:

They are all matter.

Here's why:

* Matter is anything that has mass and takes up space.

* Air is a mixture of gases, primarily nitrogen and oxygen, and it has mass and takes up space.

* Helium gas is a pure element that is a gas, and it definitely has mass and takes up space.

* Salt water is a solution of salt dissolved in water. Salt and water both have mass and take up space, and so does the solution.

* Copper wire is a solid metal, and it has mass and takes up space.

Let me know if you'd like to know more about the properties of these substances!