Pepsi volcano

The secret is in the texture of the Mentos which allow carbon dioxide (CO2) bubbles to form extremely rapidly. Each Mentos candy has thousands of small pores on its surface which disrupt the polar attractions between water molecules, creating thousands of ideal nucleation sites for the gas molecules to congregate. In addition to that, the gum arabic and gelatin ingredients of the Mentos, combined with the potassium benzoate (food preservative), also help in this process by lowering the surface tension of the liquid and allowing for even more rapid bubble growth on the porous surface of the Mentos.

Factor that contributes to the size of the geyser is how rapidly the object causing the foaming sinks in the soda. The faster it sinks, the faster the reaction can happen, and a faster reaction creates a bigger geyser. The temperature of the soda also factors into geyser size. Gases are less soluble in liquids with a higher temperature (warmer soda = bigger geyser).