Alan Alda (best known as an actor but also a passionate supporter of science) and the Center for Communicating Science at Stony Brook University (CCS) challenged scientists to explain what a flame is to an eleven-year-old. This simple question elicited over 800 written and video responses and was judged by over 5000 eleven-year-old kids.
The winning video (seen below) was submitted by Ben Ames, a graduate student in quantum optics at the University of Innsbruck.
You can peruse the other finalists and honorable mentions, plus see an interview with Ames, here.
By the way, the CCS, which Alda helped to found, is asking ten to twelve year old children to provide the next challenge question. If you know a kid with a question, he or she can follow the links to submit it.