Coke and Mentos Powered Car
An explanation of how it works, from their YouTube video:
"The Coke Zero & Mentos Rocket Car uses a piston mechanism: a six-foot long rod sits inside a six-foot long tube attached to each bottle of Coke Zero. When the Mentos drop into the soda, the pressure tries to push the rod out of the tube. With 108 rods all pushing at once, that gives us a lot of power.
All that power is pushing against a wall braced with 3,600 pounds of cement blocks. So all the force is directed into moving the Coke Zero & Mentos Rocket Car forward. We get one big push for six feet, and then it's all coasting from there."