First, gather all materials.
Worx toilet bowl cleaner (20% hydrogen chloride) Unfortunately, I'm not sure they produce it in this concentration anymore :( I went to purchase some for a class experiment and all they had was the watered down version. No fun!! I'll have to check a few other vendors.
250 mL Flask
9 inch balloon
Matches (now you know this is going to be awesome!)
You may want to have a fire extinguisher nearby just in case.
See why in the video (Filmed with the GoPro2... the room doesn't fill with smoke, it's just a little condensation on the lens. Check out step 4 for more video action).