Calcium Carbide + H20 makes Acetylene gas, which is lit by a torch. You can find hundreds of videos of people doing this on youtube. Would be easier to do for real than to fake.

Acetylene gas makes a much louder bang for your buck in a test like this. ; )

From an unlit oxy/acetylene rig, fill a ballon with a mixture of the gases, tie a string to it and set the string on fire (obviously, the more inventive among you can come up with better ignition devices, this is just the most simple).

If you live in a populated area, prepare for the bomb squad to show up. =)

Oh yeah, you might want earplugs. X D