Vinegar is an acid; in water, it breaks up into hydrogen ions (protons), and acetate ions. Baking soda is sodium bicarbonate, a salt that, in water, breaks up into bicarbonate ions and sodium ions. The bicarbonate ions can combine with the hydrogen ions, to create carbonic acid, and back again because it *is* an acid. However, the carbonic acid can also break up into carbon dioxide gas and water. The carbon dioxide can build up to levels where the water can't contain it anymore, and it bubbles out of solution. Being a gas, it takes up a great deal larger volume than the solution containing it, so it will swell up and come over the top of any container you perform the reaction in.