The science behind the Coke and Mentos reaction
When Mentos drop into soda, a fizzy fountain erupts because the candy's bumpy surface creates lots of tiny spots for carbon dioxide gas to make bubbles, which quickly push the soda out of the bottle like a geyser. The soda's ingredients also help by making the liquid a bit "slippery" for the gas, so the bubbles can escape even faster.
This was a really exciting experiment and 5th class had to go outdoors to test it. There were some good fizzy fountains and a bubbly, minty mess was left afterwards but it was great fun.