Materials:
> Pen
> Plastic / Bottle Cap
> Wall Tack / Hot Glue / Clay (not in picture)

Tools:
> Lighter
> Pliers
> Scissors
> (maybe a small rod like a skewer)

Step 1:

Step 1
> Grab your pliers and pen!

Step 2:

Step 2
> Dismantle the pen and keep the cap, ink, and housing!
> (You may need a wooden rod to push out the small end cap)
> Only throw out the end cap!

Step 3:

Step 3
> Use the bottle, cap or plastic, and lighter.
> In this case I am using the plastic because I threw away the cap on accident!

Step 4:

Step 4
> If you are using the cap then just screw it on tight.
> If you are using plastic, fuse it together by lighting the plastic and (using pliers or a glove) press the hot plastic to the bottle
> You may also hot glue it down!

Step 5:

Step 5
> Heat up the scissors or knife and melt the plastic / cap
> The hot metal melts the plastic and allows you to cut with out cracking the plastic.
> (You may want to use the pliers or gloves if you touch the metal)
> Make the hole slightly smaller than the pen housing.

Step 6:

Step 6
> Using hot glue / wall tack / or clay to seal the pen housing in the hole
> Blow throw the pen and feel for air leaks and repair as needed!
> Repairing the leaks will just be the process of gluing the hole or covering it with clay / wall tack!

Step 7:

Step 7
> Place the cap on the pen and stomp on the bottle like in the picture at the begging!

Step 8:

Step 8
*MOD*
> This is my modded pen cap
> Using the lighter I melted a hole at the top of the cap and placed the ink in the cap
> Then I fused the ink to the cap and fused the end shut so ink wouldn't come out!
*UPDATE*
> The ink can be removed completely allowing the missile to go farther!
> Make sure there are no places for the air to leak in the rocket / missile as well or the pressure will escape and the result is a 4 foot launch!