And while we're tangentially off-topic, I will definitely vouch for the Diet Coke + Mentos reaction. Some friends and I met up before going on a birthday trip to Kings' Dominion (amusement park) specifically to try it out. We took a 2-liter bottle of Diet Coke, and the birthday girl took a needle & thread and strung together all the mints from a Mentos pack. She ran the camera while I dropped the Mentos into the bottle.
Sitting on the ground, that sumbitch sprayed a fountain of suds up higher than me, and I'm just over six feet tall. It ROCKED.
The Mentos-on-a-thread idea was the best that she could figure they did on that often-viewed Bellagio-style video. Yank a trigger string to drop 'em all in at once, and the reaction is that much quicker.