Here's how it works:

* Phenol reacts with formaldehyde in the presence of a base catalyst (like NaOH).

* The formaldehyde undergoes electrophilic aromatic substitution with the phenol.

* The initial product is a hydroxymethylphenol, which undergoes further reaction with formaldehyde to form p-hydroxybenzaldehyde.

The reaction can be summarized as:

Phenol + Formaldehyde → p-Hydroxybenzaldehyde

This reaction is important in organic chemistry for the synthesis of various aromatic compounds, including pharmaceuticals and dyes.