Did I say it was very simple!!
Step 1: Materials
2. A dowel that fits in, and is longer than the tube(minimal space in between the inner wall and dowel)NOT A SKEWER LIKE IN THE PICTURE.
3. Two(2)small soaked peices of paper towel(toilet paper preferably)
that fit tightly in the pipe.
Step 2: Firing
Take the dowel and ram it into one end very quickly, pressure builds up between the balls and one comes out.
If you get the seal right the peice will come out with a bang but with not much force. Perfect for kids (If their younger than 4 the bang will scare them).