My first cannon was cans taped together with aluminum foil duct tape, the bottom can being the combustion chamber. Rubbing Alcohol was the propellant. Lacking the judgment to find a proper range to fire it, a suitable cross street was chosen. As the match was applied to the weep hole a police car was passing by a block away. Unfortunately he slowed to see what was all the fuss and the tennis ball hit the door right bellow his window. We scattered like roaches. After he rounded us up and gave us a lecture. He inspected the cannon and ask us to give him a show.