Diet Coke & Mentos
Diet Coke & Mentos: the classic, foam-inducing combination. For my nephew Henry's 4th birthday, I bought him a special contraption ("Steve Spangler's Geyser Tube") which allows the rapid plunging of a whole stack of Mentos into a 2 Liter bottle of carbonated beverage, resulting in a 25-foot geyser of foam that spews forth. We conducted the experiment just before Thanksgiving dinner, and overall the audience was impressed. My brother-in-law Tim assisted in the effort.
Of course, this is nothing compared to these guys: