Giant Air Cannon Experiment

Materials needed:
Cardboard box (12 by 12 by 12 inches works fine, but the dimensions aren't critical);
Plastic sheet, such as heavy-duty trash bag material;
Packaging tape;
Hydrochloric (Muriatic) acid, HCl;
Household ammonia, NH3OH solution;
Paper towels;
Gloves.
Tools Needed:
Box cutter;
Marking pen;
Tape dispenser;
Circle template (pie plate, etc.).
Construction of the cannon is simple:
Cut the four flaps from one end of the box.
Fold the flaps on the opposite end closed.
Mark these flaps with a circle whose diameter is about half the length of the side of the box (6 inch hole for the box in our example. Cut out the circle in both layers of flap.
Tape the flaps closed. Reinforce the circle edge with more tape.
Cut a sheet of plastic to fit the open box end with about 5 inches of excess on each side.
Tape the plastic sheet over the open end of the box. Allow plenty of sag in the sheet. Seal the sheet to the box on all sides.
Construction of the air cannon is complete.
Operating the air cannon:
Pinch the sheet in the center and pull out.
Aim the air cannon.
Slam the plastic sheet with your palm to send a blast of air toward your target.
Giant Smoke Rings

The air cannon will produce smoke rings if you fill the box with smoke before you fire. The type of smoke I use is produced by combining the vapors of hydrochloric acid and ammonia. This "smoke" is ammonium chloride and water mist.

To do this, pour a little bit of each liquid onto separate paper towels (while wearing gloves), and drop both towels into the air cannon. But watch out! Both vapors are very noxious before they combine. The acid fumes can cause damage to lungs and eyes. Keep your face away from the opening of the cannon, and don't fire it towards the faces of others.