actaully, thats not the best way of doing it.
Its a lot better if you make the pipe hole about half way up the bottle. Also, dont use a knife, use a lighter and then when the plastic is hot enough/about to melt, stick your hollow pen into it and it will make a perfect size hole....
Then make the tin foil bowl around the top of the pen. Add a shotgun up top somewhere. Pour in water, and boom, you have a water bong.
Here, like this....
