Cobalt chloride paper will turn pink when it comes into contact with water vapor.

Here's why:

* Cobalt chloride is a chemical compound that exists in two forms: anhydrous (without water) and hydrated (with water).

* Anhydrous cobalt chloride is blue.

* Hydrated cobalt chloride is pink.

When the blue, anhydrous cobalt chloride paper encounters water vapor in the air, it absorbs the water molecules and forms the hydrated pink form.

This color change makes cobalt chloride paper a useful indicator for detecting the presence of water vapor.