I wonder why foam will form when you react NaOH and acetic acid together. Thanks!?

Yes, everybody is wondering about that, because no one else has seen it.

How do you know it was NaOH?

When acetic acid (CH3COOH) and baking soda (NaHCO3) react, they form carbonic acid (H2CO3) and sodium acetate (NaCH3OO). The carbonic acid will not stay in that form in solution because it's unstable. It will further decay to produce liquid water and gaseous carbon dioxide. The CO2 bubbles are heavier than our air, which means they will collect and bubble over.