Materials: medicine bottle, drill, pen tube, hairspray, thumbtacks, sparker from a lighter, potato.
Step 1: Take the medicine bottle, drill a hole in the non cap end.
Step 2: Stick the pen tube in the hole and put glue all around were it meets the medicine bottle.
Step 3: Stick thumbtacks into the medicine bottle and have coming towards eachother, but not contacting eachother inside the bottle.
Step 4: Wrap the ends of the wires from the lighter sparker around the thumbtacks.
Step 5: Stick the potato into the top of the pen tube to wedge a piece of potato into the pen tube.
Step 6: Spray your hairspray into the medicine bottle, close the bottle quickly to prevent losses, and press the sparker to ignite the hairspray mixture and send the potato flying!