Why does mixing baking soda and vinegar cause fizzing?

When baking soda (sodium bicarbonate) and vinegar (acetic acid) combine, they undergo an acid-base chemical reaction. The acetic acid reacts with the sodium bicarbonate to produce carbonic acid, which is unstable and immediately breaks down into water and carbon dioxide gas. The fizzing you see is that carbon dioxide being rapidly released as bubbles. The reaction can be summarized as: sodium bicarbonate plus acetic acid yields sodium acetate, water, and carbon dioxide. The vigorous bubbling happens because the CO2 is produced faster than it can dissolve into the liquid, so it escapes as gas. This is the same basic principle behind carbonated beverages, just happening spontaneously through a chemical reaction rather than pressurized infusion.