This year, for Van Dyke's Sports Camp VBS, we're doing a more general "Science Olympiad" instead of just Robotics. Tonight we played with Diet Coke and Mentos. I built a half-dozen nozzle rigs like this. The one in this video has a diagonal slot cut into it. The pressure from the Diet Coke was enough to knock the rig over and the top of the nozzle popped off. Then the bottle took off across the parking lot. It was cool enough that we spent the rest of the time trying to get another bottle to take off horizontally, without much success.