When you add phenolphthalein to a sodium hydroxide solution, the solution turns pink.

Here's why:

* Phenolphthalein is an indicator, which means it changes color depending on the pH of the solution.

* Sodium hydroxide (NaOH) is a strong base, meaning it has a high pH.

* In basic solutions (high pH), phenolphthalein turns pink.

So, the pink color indicates that the solution is basic due to the presence of sodium hydroxide.