What is in soda that makes a geyser?

Mentos candies contain tiny holes on their surface called nucleation sites, which provide ideal places for carbon dioxide bubbles to form. When submerged in soda, the Mentos candies create so many nucleation sites that large bubbles form rapidly. This causes a surge in pressure inside the bottle that's strong enough to propel a liquid spray from the spout.

The eruption of soda is a result of the release of pressurized carbon dioxide gas. The carbon dioxide is dissolved in the soda under high pressure, and when the pressure is released, the gas rapidly expands and forms bubbles. This causes the soda to foam up and erupt from the bottle.

The geyser effect is most pronounced with diet soda, because diet soda contains more carbon dioxide than regular soda. The higher carbon dioxide content creates more bubbles, which leads to a more powerful eruption.

You can also create a soda geyser with other objects that have nucleation sites, such as sugar crystals or salt crystals. However, the geyser effect will not be as strong as with Mentos candies, because these objects do not have as many nucleation sites.