Here's why:

* pH paper works by a chemical reaction. The pH paper contains a chemical indicator that changes color depending on the concentration of hydrogen ions (H+) in a solution. This reaction requires the presence of water.

* Water is necessary for the indicator to dissolve and react. Without water, the indicator won't be able to change color, and you won't be able to determine the pH.

What you can do:

* If you need to test a substance that is not a liquid, you can use a small amount of water to dissolve a sample of the substance and then use the pH paper.

* Some pH test strips are designed to be used on solid surfaces. These strips contain a moisture-sensitive indicator that reacts directly with the surface, without needing additional water.

Important: Always refer to the instructions on the pH paper package to ensure you are using it correctly.